Write the report until 5 pm.
Write the report by 5 pm. 

Write the report until 5 pm means “Start writing the report now, continue writing it until 5 pm, then stop writing it regardless of whether it is finished or not.”

Write the report by 5 pm means “Make sure that at 5 pm the report is written and finished”. This example makes more sense than the first one.

As a preposition of time by means on or before.
E.g. We will send you the documents by tomorrow.

• Use “by” when you refer to a deadline.

Until means up to a particular time.
E.g. We have until tomorrow to send out the documents.

• Use “until” when you refer to the period of time before a deadline.

By is used to describe an action or event that happens on or before a particular moment.
E.g. We expect to know the judgment by the end of June.

Until is used to describe a state or situation in the period of time up to a particular moment.
E.g. The court proceedings are expected to last until June.

You may think of until as referring to the moment a state or situation changes.
E.g. Comments on the draft may be submitted until March 2nd.

Here are some more examples:

1. I can do it by 3 pm.
             Now   → do it and finish it →  3 pm (it will be finished)
2. I can do it until 3 pm.
             Now   →       do it       →  3 pm (I’ll stop even if I haven’t finished)
3. I can’t do it by 3 pm.
             Now   → do it / not do it →  3 pm (either way it won’t be finished)
4. I can’t do it until 3 pm.
             Now   →  can’t do it  →  3 pm (I’ll start)

QUIZ

Fill in the sentences with by or until:

1. I need to do the laundry ______ 7pm.
2. They have ______ March 3rd to move out of their apartment.
3. We can play ________ he comes home.
4. My friends are supposed to come _______ next week.

ANSWERS

1. by
2. until
3. until
4. by

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