Commas have two main uses: to separate lists of items, and to separate some clauses from the main sentence. Commas often (but not always) correspond to a pause when speaking.

Commas to separate lists

When we want to combine two words in a list we use a conjunction between them. When we want to combine three or more items, however, we only use a conjunction between the last two words, and separate the other words with commas:

Birds and bees can fly.

Birds, bats and bees can fly.

Some people will put an extra comma before the conjunction. This is called a "serial" or "Oxford" comma. Either way is fine, as long as you're consistent.

Birds, bats, and bees can fly.

Sometimes, for emphasis, people will use multiple conjunctions instead of commas. This famously happens in the movie "The Wizard of Oz":

Lions and tigers and bears, oh my!

When playing details, never forget your commas! You can add them to your card at the beginning or end of your word, depending on where you want to place it in the sentence. For example, we can add , zombies to a sentence like this:

I invited robots, zombies, and pirates to my birthday party.

Or we could add zombies, like this:

I invited zombies, robots, and pirates to my birthday party.

Commas after subordinate clauses

When a sentence has a main clause and a subordinate clause, if the subordinate clause goes first it is followed by a comma.

To review subordinate clauses, read this. As far as punctuation, though, just remember that you can write

I bought a new car because I had lots of money.

or

Because I had lots of money, I bought a new car.

But if you choose the second way (with the subordinate clause first), don't forget the comma.