What's the difference between by and until?

by = before a deadline
– I need to finish my reports by five o’clock. 
– I need to finish my reports before five o’clock. 

by = on or before a deadline 
– The package should arrive by Friday. 
– The package should arrive on or before Friday. 

Until = from a time to a deadline
– I’m going to work on my report from now to five o’clock. 
– I’m going to work on my report from now until five o’clock. 
– I’m going to work on my report until five o’clock. 
– Until you arrive, we can’t start the meeting. 
– I’ll wait at the station until you arrive. 

Note: till = until 
This is usually a bit more casual. 
– I’ll be here till about five. 
– Can you wait till I get back? 

Practice examples: 

1. I’m working  _____ six.

2. I’m at work now. I’ll finish work at six. I think I’ll be home _____ seven.

3. I’m at home now, but I’m going out later. I’ll be home _____ seven.

4. No dessert _____ you eat your vegetables.

5. _____ ten o’clock, I want to be in bed.

6. This offer is only available _____ Saturday.

7. Construction will continue _____ next summer.

8. Homework must be submitted _____ tomorrow at the latest.

9. I stayed up _____ four o’clock this morning.

BONUS CHALLENGE 
10. The train won’t get to the station _____ nine.

Answers: 
1. until     2. by     3. until     4. until     5. by     
6. until     7. until     8. by     9. until

10. both ok
The train won’t get to the station by nine. = not before nine
The train won’t get to the station until nine. = from now to nine the train will be out of the station