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Prepositions after verbs

Complete in vs. complete by

Use complete in for duration and complete by for deadlines or time limits.

Quick answer

Time expressions with complete

Use complete in to show how long something takes. Use complete by to state the deadline or finishing time.

They completed the project in three months.
Please complete the form by Friday.

Choose the pattern by meaning

These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.

1
complete in + period of time

Showing duration

Use in to say how long it takes to finish something.

She completed the course in six weeks.
The builders finished the work in two days.
2
complete by + deadline

Giving a finishing point

Use by for the latest acceptable time.

We must complete the report by tomorrow morning.
Students should complete the task by 5 p.m.

See the difference in context

The same subject can take different constructions because each sentence answers a different question.

Construction
Example
Meaning
complete in
He completed the race in under an hour.
state the amount of time needed
complete by
Please complete the application by Monday.
state the deadline
complete within
The repairs were completed within a week.
emphasise that something finished before a limit

Common mistakes

Some combinations appear in real text but do not mean what learners often assume.

Using by for duration

By refers to a deadline, not the length of time needed.

✕ She completed the book by two weeks. ✓ She completed the book in two weeks.

Using in for a deadline

In expresses duration, not a final time limit.

✕ Please complete the form in Friday. ✓ Please complete the form by Friday.

Quick practice

Choose the expression that matches the intended meaning.

1. The team completed the project ___ four months.
2. Please complete the survey ___ Friday evening.
3. He completed the exam ___ ninety minutes.

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Frequently asked questions

What is the difference between complete in and complete by?

Complete in refers to duration, while complete by refers to a deadline.

Can I say completed within a week?

Yes. Within a week means before the end of that period.

Is complete by tomorrow correct?

Yes. It means the task must be finished no later than tomorrow.

calculate, by, calculate, in, calculate, from "call for" or "call on"? "campaign for" or "campaign against"? cancel by, due, at, in or for? capitulate to, in, like, by or after? captivate by, for, with, from or in? "capture by" or "capture in"? "care about" or "care for"? carry with, by, in, to or into? carve into, out, in, on or with? cast in, into, for, by or on? "catch in", "catch by" or "catch on"?

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