You can only replace 'ago' with the word 'before' in reported speech or in a text in which 'before when' is clear. Usually the tense of the verb will be past perfect. Direct speech: I did it a few hours ago. Reported speech(Indirect speech): He said he had done it a few hours before.

I agree with both Wolvets and deep blue. Sarah met Julie in April but she met Max in January. = She met Julie 2 months ago. = She had met Max four months before. (We use the past perfect to talk about an action which happened before another action in the past)

Well, well Both refer to a time in the past. Some time ago means some time back from now. Some time before before means some time back from then. Someone is writing a letter home when they are on holiday: There has been a lot of rain during the week. It started raining again two hours ago but the people who have been on holiday here for two weeks told us that it had rained before

"Ago" is an adjective, so use it tell more about a noun: a long time ago twenty years ago a century ago "Before" is an adverb, so use it with a verb: Before you leave, wash your hands. / Wash your hands before you leave. They practiced before they sang. She went before he did. I will pray before I eat.