A hidden creature is “unseen and unheard” (and unsmelled, untouched and untasted) - this is different (and better) than merely being unseen.

An unseen creature is “known” to everyone and can be freely targeted (except by things that require the target to be seen) albeit at disadvantage. There is no mystery about their location on the battlefield.

A hidden creature, however, is only “known” to creatures that have perceived them: by having a high enough passive Perception or by creatures that have taken the Search action and succeeded on an active Perception check. If you want to target them you have to guess where they are - if you guess wrong, you miss, if you guess right you can attack (with disadvantage). This might be easy (he’s in the wardrobe) or hard (she’s somewhere in that 60 foot radius sphere of darkness).

There are other subtleties. For example, if you are frightened of an unseen opponent you can’t approach them because you know where they are, if they are hidden from you you can approach them (not that you would know you are).

“Giv[ing] away your location “ means that you are no longer hidden. You may still be unseen if, for example, you are heavily obscured or remain invisible.

"Giving away your location" just means that enemies know where you are. It doesn't necessariamente mean they can see you.

For example, you can be invisible but, due to drastically failing a Stealth check by sneezing, the enemy now knows there is an invisible creature standing somewhere by the bookshelf. Thus giving them something to aim at (despite not knowing the exact disposition of your body or whether you are armored or holding a shield or weapon etc).

If you were hiding using conventional means (say, in a shadow behind a wall) and then jump out and attack then enemies will now know your location and will be able to see you. Unless you duck back behind the wall of course, but even then they will know you are lurking around somewhere nearby. If you are a rogue you would get sneak attack on that first attack, but possibly not subsequently since enemies know you are around and will be on the look out. (The DM may allow you to make another Hide check to fool your opponents).

Regardless, your location has been given away and an enemy may just decide to walk around the wall to hit you.

If you are invisible and start without the enemy knowing you are there thanks to a successful Hide check - and let's say its Greater Invisibility, so attacking doesn't break the invisibility - then attacking "gives away your location" because, again, enemies know you are around and roughly where you are. You may be invisible, but if the enemy suddenly finds themselves with an arrow sticking out then they vontade know somebody is about ("the arrow came from over by that tree!").

You are still invisible so enemies attack with disadvantage, but they know roughly were to aim at unless you move. If you do move then the DM may require further Stealth and/or Hide checks to avoid the enemy being able to pinpoint your location now that they know an invisible enemy is around (whereupon, if they fail a Perception check, they will just have to guess your location and any attack may just miss anyway as per the rule you quoted).

Respectively: Yes. No. Yes. Or rather;

Hiding only causes you to be unseen (and gain that sweet disadvantage) whilst you do nothing to make yourself ineligible for hiding, and then stops until you make another Hide action.

When you try to hide, make a Dexterity (Stealth) check. Until you are discovered or you stop hiding, that check's total is contested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence.

When we hide, we make a Stealth check that lasts until we stop hiding or are discovered. Logically, that then returns us to our normal state: Our enemies can perceive our location because they don't need to make a Perception check to do so, so they don't get disadvantage on their attacks against us. (Unless some other effect is in play).

So

What makes you illegible for hiding?

The DM decides when circumstances are appropriate for hiding.

The rules then give the DM some guidance on that: you can't hide if the enemy can see you clearly - you can't make noise, you can't approach a creature, unless the DM decides to rule that it's distracted and...

Basically, the circumstances under which you can hide or not hide are all down to the situation and your DM (which I find, as both a player and a DM, to be incredibly annoying to discuss/argue at the table, but here we are.)

To break down your questions:

Does this mean enemies attacking the hidden creature no longer have disadvantage, as described in the preceding paragraph?

Yes. We gave up the benefits of our Hide action (the Perception check to find us) when we made our attack.

If the attacker then breaks line of sight and becomes unattackable...

(Notably at this point, presuming we've run behind a building or something; we're both out of line of sight AND unseen. If an enemy could shoot a magic spell through the wall, (or volley an arrow over it) ignoring that line of sight problem, but couldn't see through it, we'd get that sweet disadvantage bonus again.)

...then becomes partially visible the following turn, do enemies attacking the hidden creature once again do so at disadvantage?

Nope, not unless we took the time to do a Hide action behind that wall, at which point the enemies might not have noticed us becoming partially visible again; depends how our DM feels about how concealed we are.

Para resumir:

The Hide action allows us to conceal our location. Essa tb causes us to be "unseen"; a rule that happens to overlap with some other things that make you unseen (e.g. invisibility or being behind something).

Attackers have to guess our location to attack us. How the DM guesses a monster's attack for a situation they know everything about is left up to their discretion.

Revealing our location in any way, ends that Hide action, because the Stealth check to conceal us only lasted until we did so.

Termo aditivo:

"I've hidden behind this 5x5 cube, so enemies don't know my location."

Sure, you could make that hide check now whilst your enemies are the other side of it. They don't know your location, so would have to guess it to attack you.

On the other hand, if an enemy walks to the other side of said cube, and you're just... standing there with no further cover. You're clearly visible. You're discovered, and your Stealth check ends.

"I've hidden behind this 1x1 tree, so enemies don't know my location."

Sure. But I'm about to guess that you're still behind that tree, since you haven't run into the open. I'll take the disadvantage to keep shooting you.

OR

"I'd like to hide behind this 1x1 tree."

Your enemies just watched you run behind that tree, which isn't wider than you. You cannot make a Hide check.

If you attack while hidden then your location becomes known immediately afterwards. This just means everyone now knows where you are, or more specifically, where you were when you made the attack. It doesn't necessarily mean you are now visible. You might have been hidden because you were lurking quietly in a dark area which no enemy could see into. Or you could be under the effect of a Maior invisibilidade spell. If anyone attacking you still can't see you, they get disadvantage on any attacks.

There is no real "partially visible" in 5th edition D&D. You could have cover because you are hiding behind a tree or because you are in an lightly obscured area eg. light fog. It doesn't matter. Enemies can either see you or they can't they get disadvantage on their attack rolls if they can't. If you have cover then you get an AC bonus in most cases.

The difference between being hidded and unseen

The disadvantage on attacks is only for being unseen. The main difference between unseen and hidden is that, for the latter, enemies don't know your exact location. If your location is known but you are unseen then enemies can try to attack you but their attacks get disadvantage. If you are hidden they also get disadvantage but they would have to guess at your location before making an attack. If they guess incorrectly they attack where you are not and thus you(the PC) are not attacked.

If you moved behind cover e use the hide action então you'd be hidden from everyone unless their passive Perception check beats your Stealth roll. T

• To illustrate: you might try to hide but the noise of your armor and the sound of your breathing is too loud so they can hear you. They might not be able to see you, but they can locate you via your breathing. Not being able to see you might be a temporary situation as well. If you are hiding behind a tree they may be able to see you simply by walking around it. If you climbed up the tree and hid in the branches they might have a tougher time of it.

  Keep in mind, your enemies aren't (generally) complete idiots. If you pop out from behind a tree, shoot at them, then duck back behind again and hide they are going to have a fair idea where you are. They might need to search a bit if you've crawled under a pile of leaves, but they will still be aware that you are around and search for you or ready an action to shoot at you if you reappear.

With the new questions and mention of context, here is what I hope will be a more concise answer, but first I should elaborate on the key three rules you are looking at, as I believe there is some confusion there with how they all interact:

The "Hiding" rule

The DM decides when circumstances are appropriate for hiding. When you try to hide, make a Dexterity (Stealth) check. Until you are discovered or you stop hiding, that check's total is contested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence.

This part of the rule clearly states that it's up to the DM to decide when you can, and can't, attempt to hide. It also points out that after you make a Stealth check, that is contested by the Perception check of any creature actively searching for you.

You can't hide from a creature that can see you clearly, and you give away your position if you make noise, such as shouting a warning or knocking over a vase. An invisible creature can always try to hide. Signs of its passage might still be noticed, and it does have to stay quiet.

The part referencing that you can't hide from a creature that can see you clearly may answer your first question about giving away your location, as that concept would refer to you being clearly visible, making an audible noise, or anything really that the DM considers giving away your location.

In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the DM might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack roll before you are seen.

The first part of this final paragraph of the rule states that most creatures would be aware, therefore if you were hiding in a bush, or behind a wall, and you suddenly moved out from behind the object or obstacle, you would be making yourself visible once more. I would rule that this is overt enough it does not require a check, however, it could be argued that if you are doing this behind someone to attack them, it's surprise, and if it's not to attack them but rather to move, it would prompt a Perception check per the first paragraph's explanation.

As a point of addition, there are many MUITOS conditions that go into whether or not a creature is visible or not beyond them hiding, as mentioned in the rule itself:

What Can You See? One of the main factors in determining whether you can find a hidden creature or object is how well you can see in an area, which might be lightly or heavily obscured, as explained in chapter 8.

The "Hide" action

When you take the Hide action, you make a Dexterity (Stealth) check in an attempt to hide, following the rules for hiding. If you succeed, you gain certain benefits, as described in the "Unseen Attackers and Targets" section later in this section.

As you can see, the "Hide" action is simply the method in which you enter hiding, all the rules for the "Hide" action are described in the "Hiding" rule, rather than the "Hide" action itself, the benefits gained from "Hiding" are the benefits gained from successful use of the "Hide" action, which are yet to be explained...

The "Unseen Attackers and Targets" rule

Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.

Mention of "hiding" may very well be a reference to the "Hiding" rule and "Hide action".

When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. If the target isn't in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target's location correctly.

This section focuses on what happens with creatures attacking an unseen target, or you if you are using the "Hide" action successfully, so all the effects and rules for that condition have already been covered and are based on those rules and not the rules purely discussed in the "Unseen Attackers and Targets" rule.

When a creature can't see you, you have advantage on attack rolls against it. If you are hidden--both unseen and unheard--when you make an attack, you give away your location when the attack hits or misses.

This section focuses on what happens when you are unseen, or "Hiding" and attack a creature. To clarify on the rule a bit, as you make the attack, you are still considered to be "Hiding" and therefore benefit from the advantage, you give away your location after you've made the attack, regardless of whether it hits or not.

The Questions

If you're still wondering about those questions and why I just unloaded those rules on you, it's mainly to give you context to my shorter answers that will make reference to them:

1. Giving away your location can mean many things, I would refer to the "Hiding" rule and the "What can you see?" segment for that. If the location no longer has to be guessed, there is no disadvantage in play and it is a wholly ordinary attack process. There are no other implications beyond losing the benefits or conditions granted by these three rules mentioned.
2. If you change location, you are likely going to have to reference the "Hiding" rule for confirmation as to whether you are no longer considered to be hiding or not. This is a case by case thing and is ultimately up to your DM.

I hope this answer has explained all you need and I'll provide further clarifications if I've been unclear.