Hide action in Combat
Gaining the Hidden condition:
You can only attempt to hide if the opponent can't see you. You need total cover (lightfoot halfling can hide behind allies), heavy obscurement (wild elf can hide in natural light obscurement, those with the Skulker feat can hide in light obscurement), invisibility, or a DM ruling that the opponent is distracted enough not to see you.
An invisible creature can’t be seen, so it can always try to hide. Signs of its passage might still be noticed, however, and it still has to stay quiet. Foes have disadvantage to notice you, since you cannot be seen.
If you meet the conditions above, you may roll Stealth to gain the hidden condition. This means enemies cannot determine what square you are standing on.
You have disadvantage on stealth checks to hide again in the same position after revealing your position.
Remaining Hidden:
You remain hidden as long as you are in any kind of cover or concealment. If you make noise (such as shouting a warning or knocking over a vase), you give away your position.
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 Dungeon Master might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack before you are seen.
You retain the hidden condition until you are fully visible or attack. This means you can attack from hiding using ranged attacks (and gain advantage) but not when coming out of cover.
If you attack, you lose your hidden state after the attack (unless you have the Skulker feat).
Common Questions:
Are you immediately spotted when you leave cover/concealment, or do you have time for an attack? You can attack from cover/concealment with advantage, but lose the advantage if you leave the area with cover/concealment.
Do you qualify as hidden if monsters know where you're hiding but can't see you? Yes, although you have disadvantage to Stealth in these conditions.
How much cover is enough cover to hide? You need total cover, unless you are a lightfoot halfling (then a ally is also acceptable)
Can you hide while obscured if you're still in a bright room? Yes, you can hide in heavy obscurement, unless you are a wild elf in natural light obscurement or have the skulker fear and are in light obscurement.
Example: Lightfoot Halfling is next to one of his own medium-sized party members in a room, using a Shortbow, and fighting an enemy:
Cunning Action: Using his Cunning Action to Hide behind his own medium sized party members (in conjunction with Naturally Stealthy which allows you to hide when obscured by a creature that is at least one size larger than you).
If the halfling does this trick in plain sight of enemies, he has disadvantage on his Stealth check. This would best be done before entering a room and being visible to foes.
(assuming he succeeds at Stealth Check) Normal Action: from behind party member shoot the Shortbow at the enemy with Advantage from hiding (which would reveal him), and, assuming he hits the enemy, get sneak attack damage because the Shorbow attack had Advantage.
Rogue level 3 (Dex 16) sneak attack: +5 to hit, 1d6+3+2d6 damage.
Move Action: move up to 25 ft with his movement to anywhere else he wanted.
Best stealth races:
Lightfoot Halflings: Lucky, Naturally Stealthy, halfling nimbleness
Wild Elves: 35 movement, Keen Senses, Longbow prof, being able to hide when lightly obscured by enviromental conditions, Darkvision
Tieflings: Darkvision, Darkness spell
High Elves: Minor Illusion cantrip, Darkvision, Longbow prof.,