Until and by can sometimes be confusing. Until means continuing to happen before a particular time or event and then stopping. This preposition can be followed by a noun (see examples 1 and 2 below) or it can be connecting two clauses, in which case that’s not a preposition but a conjunction (see examples 3 and 4).

1. The show will be on until the end of the month.
2. I was employed by a manufacturing company until 2019.
3. Whisk the egg whites until they look white and fluffy.
4. We’ll wait until we hear the decision.

By means not later than a particular time or date. In other words, it means before or until. By is a preposition which is followed by a noun.

1. The meeting should have finished by 4:30.
2. Application forms must be received by 31st March.
3. Please do it by the end of the day.
4. We’ll have finished by 2023.

So, when you have a clause after the preposition, the preposition has to be until, not by. But what about the times when there’s a noun following the preposition? Are until and by interchangeable then? The answer is not quite. When you talk about a deadline, use by because it means on or before. For example:

1. Application forms must be received by 31st March. (= on or before 31st March)
2. Submit the article by 3 pm.

However, if you talk about a period of time before a deadline, until is the right choice. Compare:

1. Application forms must be received by 31st March. Until then, you have plenty of time to prepare them.
2. We have until 3 pm to submit the article. I hope we can write it by then.

Use until, not by, to talk about a situation before a particular moment in time. For example:

1. We stayed at the hotel until Friday.
2. The term lasted until summer.

The sentences above are about situations that changed on Friday and in summer respectively. We use until to refer to those situation/state-changing moments.

Conclusion

To connect 2 clauses, use until. For example, Wait here until I return.

Use by for deadlines. For example, Send this email by the end of the day.

Use until to talk about the period before a deadline. For example, We have until the end of the day to send this email. We can think of what to write until the end of the day.

Use until to describe a situation or a state which changes at a particular moment. For example, I will love you until the end of time.

Practice

Click the link here and choose the right words to complete the sentences.

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