(someone) is supposed to (do something)

Say that someone "is supposed to" do something if:

• there's a rule about it
  

  I think we're supposed to shower before going in the pool.

• there's a custom that says something about it
  

  You're supposed to take your shoes off when you go into someone's house in Japan.

• you have already agreed to do it
  

  I'm supposed to pick Janelle up from school this afternoon. 

Note that "supposed to" can also be used in other situations, like when you're reporting information that you heard from other people.

bring (something) back

To "bring ___ back" means to return something. "Bring ___ back" is more casual than "return".

(do something) by (a certain time)

You use the word "by" to indicate a deadline.

You would use "by" in this way:

We're presenting this on Monday morning, so remember to get Henry's approval by Friday afternoon.

"Before" is similar in meaning to "by", but "before" doesn't carry the same meaning of "deadline". Instead, it's used when you need to do things in a certain order:

Remember to get Henry's approval before you present it to the client.

This is something you would say if a coworker is going to make a proposal to a client, but she is supposed to ask Henry, who's the Director, for permission first.