Part of an ongoing series of 5e Rules notes.

Bonus Actions are actually pretty easy, but they are not well explained in the 5e Players Handbook.

When can you use a Bonus Action?

On your turn, one of the things you may be able to do is a Bonus Action. The trick to understanding it is that you only get Bonus Actions that the rules specifically say you get. Certain rules give you a Bonus Action. You can only ever use one Bonus Action on your turn, and it can only be on your turn (you can’t use a Bonus Action in an Opportunity Attack, for example).

If you aren’t using or eligible by the rules for a Bonus Action, you don’t get one. There isn’t a “Bonus Action phase” in the turn or something. What you can do as a (single) Bonus Action has to come from a rule or ability applicable to your character.

Note that some spells have their casting time as one Bonus Action. These spells can only be used as a BA. Also, you cannot cast a Bonus Action spell if you have cast anything more than a Cantrip as your regular action.

An Example

So, for example, my Rogue, Tener, started off with only one thing I could do as a Bonus Action (the one available to everyone): a second melee attack using Two-Weapon Fighting (PHB 195).

At second level Rogue, he got the class ability Cunning Action (PHB 96), which meant I could use my Bonus Action to Dash, Disengage, or Hide.

At third level Rogue, he got the Thief archetype ability of Fast Hands (PHB 97), which meant I could use my Bonus Action for Sleight-of-Hand, disarm a trap, unlock a lock, or Use an Object.

But I couldn’t use my Bonus Action to, say, Help, because that wasn’t a Bonus Action defined for my character. I could only do those specific actions defined for my Bonus Action in my rules.

Does everyone have Bonus Actions?

Some characters don’t have any Bonus Actions, at least at lower levels (except the option of Two-Weapon Fighting, if they choose it).

When can I take a Bonus Action?

One more thing about Bonus Actions: some have prerequisites and some have none. For example:

• Cunning Action has no prerequisites. Whatever else I do on my turn, whenever I want in my turn, I can use the Bonus Action to, for example, Dash.
• Two-Weapon Fighting says “When you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand.” Therefore, you need to, in sequence:
  (1) take an Attack action with one hand; then, later in your turn, you can
  (2) use the Bonus Action to attack with the other hand.
  You can do any other allowable things in between — chat with someone, Move, etc.  But you can’t use the Bonus Action first in this case.
• Monk’s Flurry of Blows speficies “Immediately after you take the Attack action on your turn …” In order to use the FoB Bonus Action, you have to
  (1) take an Attack action, and then immediately (no Moving in-between)
  (2) use the FoB Bonus Action.

In short, if there is a prerequisite, it must be fulfilled first. If the Bonus Action says “when you do X, you can do Y,” you can’t do Y, then X.

In summary …

• The only time you get a Bonus Action is if you have a rule (usually from a class, race, or feat) that says you have a Bonus Action, and then it’s only good for what the rule says you can do with it. (And you can only do a single BA on your turn.)
• Everyone has a Bonus Action for Two-Handed Fighting (allowing you to do the second attack as a Bonus Action). That’s pretty much it.
• As a Rogue, your Cunning Action allows you to take a Bonus Action, but only to do a Dash, Disengage, or Hide. (This is a “restriction,” yes, but it’s actually granting you Bonus Actions that nobody else necessarily has. Similarly, if you take Thief, at 3rd level you can do a Sleight of Hand, disarm/unlock, or Use and Object as your Bonus Action on a turn.)

Can I take a Bonus Action to Help someone?

This came up early in my campaign. In short, unless you have a Bonus Action that specifically says you can Help on your BA, you can’t.

That said, “Help” (PHB 192) is a great Action for a character to take on their turn when they’re not sure what to do or if they don’t think their own attack on the BBEG will be effective, or if someone else will have a great attack.

For Rogues (again as it came up in my campaign) it doesn’t really come into play for allowing a Sneak Attack, though, because to Help for combat (giving  Advantage) the Helper has to be adjacent to the target — which, if they are, means a Rogue can already Sneak Attack the target anyway (PHB 96) (though a Help would let you roll Advantage on the attack, which is not for nothing).

But, again, Help can’t be done in a Bonus Action unless someone has that specifically as something they can do as a BA.