My bad.

"My bad" is a slang phrase that some people use as a light apology. It's like "Sorry!" but more casual.

When you make a really big mistake, don't say "My bad." Say "I'm sorry" or "I apologize."

"My bad" is mostly used by young men in their teens, 20s and 30s.

Didn't (do something).

When you're speaking casually, you can drop "I" from your sentence:

Oh, sorry. Didn't see you there.

I didn't realize (clause)

"I didn't realize ___" means "I didn't notice ___" or "I didn't know ___".

You use this phrase to explain why you did something in the recent past. For example, imagine that you were in the bathroom for a long time. When you finally come out, your roommate is waiting outside of the bathroom and looks angry at you. You can say:

Oh, I didn't realize you needed to use it.

"I didn't realize" is followed by a clause, which is like a sentence inside of another sentence. Sometimes the clause can begin with "that":

I didn't realize that she had gotten laid off. If I had, i wouldn't have made those comments.

But it's often left off in casual speech:

Hey! I didn't realize you were in town!

(someone) was saving (something)

When someone wants to keep some food or drinks to use later, you can say that they "are saving" it.

You can also "save" cups, plates, forks, etc. to use later.