You rented a car. It was due to be returned by 6:00 on Sunday. You just barely returned it on time. You're telling your friend about returning the car at the last minute. To set up the story, you say:
I was supposed to bring it back by 6:00.
When there is a rule, a custom, or has been an agreement for someone to do something, you say that they "are supposed to" do it:
I think we're supposed to shower before going in the pool.
You're supposed to take your shoes off when you go into someone's house in Japan.
I'm supposed to pick Janelle up from school this afternoon.
You use the word "by" to indicate a deadline.
Please get back to me by the end of the day.
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.
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