If the casting time of a spell says that it is 1 Bonus Action  when can I cast it? I'm guessing it can be used as an Action, but do I get a Bonus Action after it's casted? 

Basically, how do we tell if a spell can be casted and gives us a Bonus Action? 

On your turn you can do the following (in any order, you can also forgo any of them)

• Move (up to your walking distance)
• Perform one act with a time of an action
• Perform one act with a time of a bonus action
• Interact with an object

Some low level characters may not have any act with a time of one bonus action so they have to forego it.

So a spell that says it is a bonus action can be cast on any of your turns with the following provisos:

• You can not perform another act that takes a bonus action (as you only have one per turn)
• There is a rule that if you cast a spell using your bonus action the only spells you are allowed to perform using your action are cantrips.

Double thanks. 

I always like to say it is better to think of "Bonus action" as "Quick Action".  these are quick things you do either as an after thought (spiritual weapon) or to prepare for something (Smite spells)

I've never understood why, if a Bonus Action is supposed to be a "quick action", you cannot perform two bonus actions in a round, foregoing your main Action. 

Quote from jdahveed >>

I've never understood why, if a Bonus Action is supposed to be a "quick action", you cannot perform two bonus actions in a round, foregoing your main Action. 

 Probably something balance-wise, but if push came to shove, i'd probably allow a player to do his bonus action action as a normal action