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UPDATED: 10 JAN 20
​WHICH WORD?

By / Until

Used with "time"
for specific situations related to “time”.

by

  • not later than
    (a specified time) 

  • something could be completed before
    ​(a specific time)

until

  • up to
    (a particular time)
    ​
  • a period of (specific time) 
EXAMPLES:
​
  1. The operation will be completed by 10:00 p.m. (it might be completed before 10:00 p.m)

  2. We will arrive in Miami by 6:00 a.m. (we might arrive sooner, but for sure not after 6:00 a.m.)

  3. I’ll be there by 5. (I might arrive earlier than 5, but not after.)

  4. I hope to learn English by the time I finish the university.( maybe I will learn it before)
EXAMPLES:
​
  1. I’m going to study English tonight until 10 and then I will go out for a beer.  (from now –> 10 you will study)

  2. He will study for the test for 3 hours.  From 3:00 p.m until 6:00 p.m.

  3. I will stay at the party until it is over.

  4. I will study English until I learn it.

  5. I waited for her until 5, but she never showed up so we started the meeting without her.
NOTE: 

Usually answers the question “When” in the future tense.

  • When will the project be finished?   By November.

  • When will you arrive at the party?   By 6 for sure.

  • When will you call me?  By Thursday.  I promise.
    ​
​NOTE:  

Usually answers the question "How long"

  • How long will you be working on the project?   Until November.

  • How long will it take you to finish the report?    Until 3 this afternoon.

  • How long were you in New York?    Until it started snowing.