Actions in Combat

ATTACK

The most common action to take in combat is the Attack action, whether you are swinging a sword, firing an arrow from a bow, or brawling with your fists. With this action, you make one melee or ranged attack.   Certain features, such as the Extra Attack feature of the fighter, allow you to make more than one attack with this action.   You may also replace the ATTACK action with a TWO-WEAPON ATTACK.
  Once per round when you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you may also attack with a different light melee weapon that you’re holding in the other hand. You make BOTH of these attacks with a -2 to hit.   This replaces the standard D&D 5e Two-Weapon Attack rules.  

GRAPPLING

When you want to grab a creature or wrestle with it, you can use the Attack Action to make a Special melee Attack, a grapple. If you’re able to make multiple attacks with the Attack Action, this Attack replaces one of them.   The target of your grapple must be no more than one size larger than you and must be within your reach. Using at least one free hand, you try to seize the target by making a grapple check instead of an Attack roll: a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (target chooses). If you succeed, you subject the target to the Grappled condition. You can release the target whenever you like (no Action required), and when you move, you can drag or carry the Grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.   A Grappled creature can use its Action to Escape using a Strength (Athletics) or Dexterity (Acrobatics) check contested by your Strength (Athletics) check.  

SHOVING

Using the Attack Action, you can make a Special melee Attack to shove a creature, either to knock it prone or push it away from you. If you’re able to make multiple attacks with the Attack Action, this Attack replaces one of them.   The target must be no more than one size larger than you and must be within your reach. Instead of making an Attack roll, you make a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (target chooses). If you win the contest, you either knock the target prone or push it 5 feet away from you.  

CAST A SPELL

Spellcasters such as wizards and clerics, as well as many monsters, have access to spells and can use them to great effect in combat. Each spell has a casting time, which specifies whether the caster must use an action, a reaction, minutes, or even hours to cast the spell. Casting a spell is, therefore, not necessarily an action. Most spells do have a casting time of 1 action, so a spellcaster often uses his or her action in combat to cast such a spell.  

DASH

When you take the Dash action, you gain extra movement for the current turn. The increase equals your speed, after applying any modifiers. With a speed of 30 feet, for example, you can move up to 60 feet on your turn if you dash.   Any increase or decrease to your speed changes this additional movement by the same amount. If your speed of 30 feet is reduced to 15 feet, for instance, you can move up to 30 feet this turn if you dash.  

DISENGAGE

If you take the Disengage action, your movement doesn't provoke opportunity attacks for the rest of the turn.  

DODGE

When you take the Dodge Action, you focus entirely on avoiding attacks. Until the start of your next turn, any Attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity Saving Throws with advantage. You lose this benefit if you are Incapacitated or if your speed drops to 0.  

HELP

You can lend your aid to another creature in the completion of a task. When you take the Help Action, the creature you aid gains advantage on the next ability check it makes to perform the task you are helping with, provided that it makes the check before the start of your next turn.   Alternatively, you can aid a friendly creature in Attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally’s Attack more effective. If your ally attacks the target before your next turn, the first Attack roll is made with advantage.  

HIDING

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 such as being an unseen attacker and target.  
WHEN CAN YOU HIDE
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.

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.

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.  
PASSIVE PERCEPTION
When you hide, there's a chance someone will notice you even if they aren't searching. To determine whether such a creature notices you, the DM compares your Dexterity (Stealth) check with that creature's passive Wisdom (Perception) score, which equals 10 + the creature's Wisdom modifier, as well as any other bonuses or penalties. If the creature has advantage, add 5. For disadvantage, subtract 5.  
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 HERE.

READY

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.   First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."   When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.   When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without effect.  

SEARCH

When you take the Search action, you devote your attention to finding something. Depending on the nature of your search, the DM might have you make a Wisdom (Perception) check or an Intelligence (Investigation) check.  

USE AN ITEM OR OJBECT

You normally interact with an object while doing something else, such as when you draw a sword as part of an attack. When an object requires your action for its use, you take the Use an Object action. This action is also useful when you want to interact with more than one object on your turn.  
USE A POTION (Variant)
You may use a Bonus Action to quickly drink or administer a potion and roll for it as normal, or you may use an Action to drink or administer the potion for max healing.