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For, since, or ago
For gives a length of time, since gives the starting point, and ago tells how long before now something happened.
Quick rule
A length of time? For. The moment it started? Since. How far back from now it happened? Ago.
It goes with a length of time: for two hours, for ten years. It works with any tense.
It goes with the moment something started and is still true: since Monday, since 2015. It usually comes with the present perfect.
It comes after a length of time and says how long before now something happened: two days ago. It almost always takes the past simple.
How to choose
| Question | for | since | ago |
|---|---|---|---|
| Does it come before a length of time (two years)? | Yes | No | No |
| Does it come after the length of time? | No | SometimesSometimes | Yes |
| Does it come before a date or a moment (2020, Monday)? | No | Yes | No |
Why it trips people up
All three measure time from a different angle. For gives a length (for two years), since gives a starting point (since 2024), and ago counts back from now to a finished event (two years ago). Since and for often pair with the present perfect; ago takes the past simple.
How each one works
For
Something that lasts until now
I've lived here for ten years.
How long something finished lasted
We waited for two hours.
How long something will last
I'm staying in Lima for a week.
Since
From a year or a date
She has worked here since 2019.
From a moment in the day
I haven't eaten since breakfast.
From an event: since + clause
We've been friends since we were kids.
Ago
How long before now
I met him three years ago.
A short time back
The movie started ten minutes ago.
Asking: how long ago
How long ago did you move?
What comes next
present perfect plus for plus time: still true
He's had the same phone for five years.
past simple plus for plus time: it is over
She lived in Japan for two years.
present perfect plus since plus point in time
It's been cold since Friday.
it's been plus time plus since plus past simple
It's been years since I saw him.
time plus ago, with the past simple
I finished school two years ago.
Almost the same
I moved here two years ago.
When it happened: two years back.
I've lived here for two years.
How long it has lasted: two years, and counting.
I've lived here since 2024.
Since when: the starting point.
In a conversation
- A:
How long have you worked at the bank?
- B:
For about six years. I started right after college.
- A:
And when did you buy your house?
- B:
Three years ago. We've been very happy there since then.
Common mistakes
Wrong: It's a long time ago we spoke.
Right: It's a long time since we spoke.
After it's (been) + a length of time, the link is since, not ago.
Wrong: We've moved here a year ago.
Right: We moved here a year ago.
Ago points to a finished moment, so it takes the past simple, not have moved.
Memory trick
Count, point, or look back. A span you can count is for. A spot on the calendar is since. Counting back from today puts ago at the end: three days ago.
It depends
- Since as a reason
- Since can also give a reason: Since you are here, help me. That meaning has its own page: since, as, because.
FAQ
For or since: which one?
For takes a length of time (for three weeks); since takes the moment it started (since June). If you can count it, it is for.
Ago or before: what is the difference?
Ago counts back from now: two days ago. Before counts back from another moment in the past: She had left two days before. That second case has its own page.