Ago

The adverb ago refers to a period of time that is completed and goes from a point in the past up to now. Ago follows expressions of time:

• It happened a long time ago.
• They arrived in Athens six weeks ago.
• Not: They arrived in Athens ago six weeks.

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We normally use ago with the past simple. We don’t use it with the present perfect:

• I received his letter four days ago.
• Not: I have received his letter four days ago.

If we refer to a point in time before a specific time in the past, we use before or earlier or previously, often with the past perfect:

• We had got their invitation four days before.
• They met on the same island where they had met ten years previously.

If we refer to how long something lasted, we use for (not ago):

• When I was at school, I studied Russian for five years. (my studies lasted for five years)