I am a new DM. I do not understand how casting cantrips is supposed to work. For example: is a wizard able to cast Firebolt as an action and then, because Firebolt is a cantrip, cast it again as a bonus action?
Cantrips are spells, just like any other spell. There are 2 differences between a Cantrip and any other spell that has a level of 1 or higher:
- Cantrips do not use up a spell slot
- Cantrips can be cast on the same turn that a spell is cast using a bonus action
Cantrips still have casting times denoted just like other spells. For Firebolt
Casting Time: 1 action
Range: 120 feet
You hurl a mote of fire at a creature or object within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 fire damage. A flammable object hit by this spell ignites if it isn't being worn or carried.
This spell's damage increases by 1d10 when you reach 5th level (2d10), 11th level (3d10), and 17th level (4d10).
You'll notice that it still has a casting time of 1 action.
There do exist ways to change the casting time of spells. For example, the sorcerer's Quickened Spell would allow you to cast two Firebolts on one turn: One as a Quickened spell, and the other as a normal spell.
When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting
There is a complication to this issue, as pointed out to me by Drunk Cynic:
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
A careful reading of this section reveals that you can only use a bonus action to cast bonus action spells, and you cannot trade standard actions for bonus actions in 5e as demonstrated in this question: https://rpg.stackexchange.com/a/55902/25601
Your example spell Firebolt is cast as an action, not a bonus action. Not all cantrips are cast as bonus actions, and you can find the casting time on the spell's description. Similarly, some non-cantrip spells can be cast with a bonus action. Again, the spell's description should specify the casting time.
In most cases, you can only cast one non-cantrip spell per turn.
- If you cast a spell that requires an action (like Cure Wounds), you can also cast a cantrip that has a casting time of bonus action (like Shillelagh).
- If you cast a spell that requires a bonus action (like Healing Word), you can also cast a cantrip that has a casting time of one action (like Produce Flame).
- You can't cast Shillelagh and Healing Word both with bonus actions because RAW you only have one.
- You can't cast Cure Wounds and Produce Flame at the same time because they both take normal actions and in most cases you only get one of those per turn.
In most cases, theses are the only ways you can cast spells. Action surges and quickened spells make the process more complicated, but that's beyond the scope of this question.