WordReference Collins English Usage © 2019
You use ago to say how much time has passed since something happened. For example, if it is now Sunday and something happened on Thursday, it happened three days ago.
We met two months ago.
We got married about a year ago.
Be careful
You use the past simple, not the present perfect, with ago. For example, you say ‘He died four years ago’. Don't say ‘He has died four years ago’.
Seven years ago, she gave birth to their daughter, Nelly.
I did it just a moment ago.
You use ago only when you are talking about a period of time measured back from the present. If you are talking about a period measured back from an earlier time, you use the past perfect with before or previously.
The centre had been opened some years before.
The accident had happened nearly two years previously.
Be careful
Don't use ago and ‘since’ together. Don't say, for example, ‘It is three years ago since it happened’. You say ‘It happened three years ago’ or ‘It is three years since it happened’.
He died two years ago.
It is two weeks since I wrote to him.
➜ See since
Be careful
Don't say, for example, ‘It has been happening since three years ago’. You say ‘It has been happening for three years’.
I have lived here for nearly twenty years.
I have known you for a long time.
➜ See for
'ago' also found in these entries:

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