For, since and ago all talk about time, but each one does a different job. For tells you how long, since tells you when something started, and ago tells you how far back in the past something happened.
The pattern
I have lived in Dallas for two years.
She has worked here since March.
We moved here three years ago.
01For: how long
Use for with a length of time: for ten minutes, for three days, for a week, for two years, for a long time. For works with almost any tense, past, present perfect or future.
- I've had this cell phone for two years.
- We waited for forty minutes.
- My cousin is going to stay with us for a week.
- I lived in Chicago for five years, and then I moved to Texas.
| for (how long) | since (from when) |
|---|---|
| for two hours | since 9:00 |
| for three days | since Monday |
| for six months | since January |
| for ten years | since 2016 |
| for a long time | since high school |
| for a few weeks | since I got here |
02Since: the starting point
Use since with the moment something started: since 8:00, since Tuesday, since March, since 2020, since college, since I was sixteen.
With since, we normally use the present perfect (have lived, has worked, have been waiting), because the action started in the past and is still true now.
- I've known Sam since college.
- It has rained every day since Sunday.
- She hasn't called me since her birthday.
- We've been waiting here since noon.
03Ago: counting back from now
Ago comes after the length of time: ten minutes ago, two days ago, a long time ago. It tells you when something happened by counting back from now. The action is finished, so use the past simple, not the present perfect.
- I started this job six months ago.
- My sister called an hour ago.
- We met a long time ago.
- How long ago did you move here?
04Asking How long...?
To ask about time up to now, use How long have you...? and answer with for or since. To ask about a finished period, use How long did you...? To ask about a moment in the past, use When did you...? and you can answer with ago.
- How long have you lived in Seattle? For about a year.
- How long have you been waiting? Since 2:30.
- How long did you live in Miami? For three years.
- When did you get your license? Two years ago.
05Common mistakes
I have lived here since three years.
I have lived here for three years.
Three years is a length of time, so use for.
I live in Denver since 2021.
I have lived in Denver since 2021.
With since, use the present perfect for something that is still true now.
I have moved here two years ago.
I moved here two years ago.
Ago points to a finished time, so use the past simple.
She left before two hours.
She left two hours ago.
To count back from now, put ago after the amount of time.