How and when to use "by" and "until" can be quite confusing for people who are learning English. These similar words are both used to tell a time, but they are used in different ways.

Let's take a look at how and when we should use each of these words when speaking or writing English.

1. Use "by" to talk about a deadline. When we use "by" we are only concerned about the deadline or the time when something or some period ends.

• The students must turn in their reports by Friday.
• We need to submit this by 5 pm.
• He must finish this by next week.

We usually use the words "have", "need", or "must" when we are using "by".

• We need to leave by 9 pm.
• I have to finish my report by Thursday.
• They must hand in their proposal by next Wednesday.

2. We use "until" to talk about the period of time from a starting point until a deadline or the end of some period of time. So, "by" only refers to the deadline, but "until" refers to all the time between now and the end of something.

• I have to work until 10 pm.
• We can hand in the report any day until Friday.
• The store is open until 1 am.

"Until" can also be used in a longer form "from + starting time + until + ending time".

• I will work from 9 until 5.
• We can go from 10 am until 10 pm.
• The park is open from May until September.

We can replace "until" with "to".

• The store is open from 8 am to 9 pm.
• We can camp in the park from April to October.

3. Notice that we do not use the words "need", "have", or "must" with "until". Instead, we often use words like "will", "can", and "be verbs" with "until".

Look at the difference between these sentences.

• I can't leave until 3 pm.

This means that it is impossible for me to leave before 3 pm.

• I must leave by 3 pm.

This means that I need to leave any time before 3 pm. If I leave after 3 pm, then there will be some sort of problem or trouble.

Let's look at another example.

• We can submit the report until Friday.

This means that it is okay to submit the report any time from now until Friday. It is okay to submit now, it is okay to submit on Thursday, and Friday is the last day that is it possible to submit the report.

• We need to submit the report by Friday.

This has a very similar meaning, but this is only showing that if we do not submit this report before or on Friday, then we will miss the deadline and there will be some problem or trouble.

Summary

"By" refers to a deadline. We usually use the verbs "must", "need", or "have" when we make sentences with "by".

• We must finish by tomorrow.
• They need to inform us of their decision by next week.

We use "until" to refer to a period of time in which something happens or is possible. We usually use the words "can", "will", or any present tense verbs with "until".

• We can stay here until the store closes at 10 pm.
• I will study until 11 pm.
• We usually work until 8 pm. on Monday.

Learning the difference between by and until can help you sound like a native English speaker. It is also important to know, so other people can understand exactly what you mean when you speak English.