I saw her a month ago.
I saw her before a month.
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Can someone please explain how to use "ago" and "before"?
Is there a rule for using them?
Can I say -
I saw her one month ago.
I saw her one month before.
Are there any common mistakes that I should be aware of?
To me, your second example sentence is awkward. You saw her one month before what? The following three sentences are better:
1) I saw her one month before her birthday.
2) I saw her one month before the accident.
3) I saw her one month before she finished university.
With "before" you're talking about something in the past relative to something else in the past. So in 1), her birthday was in the past and you saw her a month prior to that.
4) I saw her one month ago. (your first example sentence)
With "ago", you're talking about something in the past relative to now. So in 4), you saw her one month before now.
I don't know if this is a common mistake, but notice how the addition of the word "for" in 5) changes the meaning: we're now talking about something that happened over a period of time in the past, relative to another past event.
5) I saw her for one month before she left forever.
So she left forever in the past, and before that, for a period of one month, I saw her. Note also that the month I spent with her probably ended quite close to the time when she left forever: I didn't see her for a month in June 2008 and then she left in May 2010.