For is used to indicate a period of time.


              - ​It rained for three days. 

               - I studied English for six years.

Since is used to refer to the starting point of actions or situations.


               - They have been married since 2005.

               - It has not rained since Monday.

Ago is used to say when past events happened, going back from today towards the past.


               - I bought a bicycle four days ago.

               - The company was founded 30 years ago.


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