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Cách dùng khác nhau giữa ago, before, previously trong câu nói về một thời diểm đã qua trong quá khứ

Word choice: Ago - Before - Previously

Use ago to say how much time has passed from the time something happened to now, the time of speaking:

   - I saw her a few minutes ago

   - We went to Madrid two years ago

Use  before to say how much time passed from the time something happened to the time in the past:

   - We went back to the same hotel where we had stayed two years before

Previously is used in the same way, but is more formal:

   - The meeting was a follow-up to one that had been held four days previously

* Do not use a preposition ( "at", "in", "on"....) before a phrase with ago:

       -They first met fifteen years ago ( NOT  at/ in fifteen years ago)

* Do not use "since' or "before' with ago:

       - I came to the USA two months ago (NOT since/ before two months ago)

* Use the past tense, not the present perfect, with ago:  

        - I started ( NOT I've started) a new job a few weeks ago