“I was supposed to bring it back by 6:00.”
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. You say this to set up the story.
I was supposed to bring it back by 6:00.
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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.
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.