Ago = before ‘now’, the moment of speaking (earlier than the present times); in the past: ‘Her plane landed ten minutes ago. In fact, here she comes now.’ ‘I saw him just five minutes ago’ (five minutes before now). ‘Two years ago I came to the US.’
Before = before ‘then’, a time in the past; previously: ‘I went to the airport last Monday to meet Sue. I hadn’t been to the airport before.’ (= before last Monday). ‘I saw him last Friday in London and two days before in Leeds.’ ‘As I said before, the work should be completed within Monday’ (Previously).
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