Time words help you make plans that actually work. When is the meeting? How long is the flight? When must the report be ready? English answers these questions with small words: at six, on Monday, in July, for two hours, since March, until Friday, by noon, during lunch. If you choose the wrong one, people come at the wrong time or wait for the wrong length of time.

One pair causes real trouble: for and since. If you say I have worked here since two years, an English speaker stops and has to guess what you mean. Getting for, since, by, and until right makes you sound careful and clear, which matters at work, at the doctor, and when you book travel.

First, learn the three words for points in time: at, on, and in. Use at for clock times and very short moments: at six, at noon, at midnight, at the weekend (in British English). Use on for days and dates: on Monday, on 3 May, on my birthday, on Friday morning. Use in for longer periods: in July, in 2026, in the summer, in the morning. A simple picture helps: at is the smallest point, on is one day, in is a bigger box of time.

Next, learn the two words for how long: for and since. For goes with a length of time, an amount you could measure: for ten minutes, for two hours, for three years. Since goes with a starting point, one moment when something began: since Monday, since 2020, since six oclock. Compare: I have lived here for five years and I have lived here since 2021. Both sentences can describe the same life; for gives the amount, since gives the start.

Now the two words for end points: until and by. Until means something continues up to a time and then stops: The shop is open until nine, so it stays open the whole time before nine. By means no later than a deadline: Please send the form by Friday. With by, the action can happen any time before the deadline; it just must not be late. Compare: I will work until six (I stop at six) and I will finish by six (I finish at six or earlier).

Finally, during answers the question when something happens inside a period: during the meeting, during the night, during my holiday. During goes with a noun, not a number. Do not say during two hours; say for two hours. If you want the length, use for; if you want the period it happened inside, use during: It rained during the night, and it rained for three hours.

One more helpful rule: English drops the preposition before some time words. We say this morning, last week, next year, every day with no at, on, or in. So say I saw her last Monday, not on last Monday.

1. 1. The train leaves at seven in the morning.

   At goes with the clock time; in goes with the part of the day.

2. 2. We have a team meeting on Tuesdays.

   On is for days of the week.

3. 3. My sister was born in 1998.

   In is for years and other long periods.

4. 4. I have studied English for six months.

   For goes with a length of time you can count.

5. 5. She has worked at the clinic since January.

   Since names the starting point of the action.

6. 6. The bakery stays open until eight tonight.

   Until means the action continues up to that time.

7. 7. Can you finish the report by Friday?

   By sets a deadline; earlier is fine, later is not.

8. 8. My phone rang twice during the film.

   During goes with a noun period, like the film.

9. 9. We waited for forty minutes at the airport.

   For plus a number of minutes shows duration.

10. 10. I have not seen him since the wedding.

    A negative sentence; since works with an event as a start point.

11. 11. The museum is closed on public holidays.

    On also goes with special days and dates.

12. 12. Do you usually rest in the afternoon?

    A question with in for a part of the day.

13. 13. Please stay in bed until the fever goes.

    Until can take a clause, not only a clock time.

14. 14. The doctor will see you at half past two.

    At again for an exact clock time.

15. 15. He slept during the whole flight to Rome.

    During says when it happened, inside the flight.

16. 16. The rent must be paid by the first of each month.

    By marks the latest possible time for the payment.

The biggest mistake is using since with a length of time. Learners say We worked since two hours, but two hours is an amount, so it needs for: We worked for two hours. Since needs a starting moment: We have worked since two oclock. Quick test: if you can count it (two hours, three days), use for; if it is a point on the calendar or clock, use since.

The second mistake is mixing until and by. Learners write Please pay until Friday, which sounds like you should keep paying again and again up to Friday. For a deadline, use by: Please pay by Friday. Use until only when something continues the whole time: The office is open until five.

A third mistake is using during with a number: I studied during three hours. During wants a named period, not an amount. Say I studied for three hours, or I studied during the evening. Also remember not to add a preposition before last, next, this, and every: say next week, not in next week.

• At a time, on a day, in a month, season, or year.
• For + amount of time; since + starting point.
• Until = it continues up to then; by = no later than then.
• During + a period noun, never a number.
• Can you count it? Use for. Is it a start moment? Use since.
• No preposition before last, next, this, and every: see you next Monday.