How long..? When..? For and Since.
When is used to say the time you start something. How long says the time you have been doing something for.
Compare When? (+past simple) and How long? (+present perfect):
· When did she begin reading War and Peace?
· She started reading War and Peace three days ago. = how long
· How long has she been learning English Grammar?
· She has been learning English Grammar for two days/since Monday.
When saying how long we have been doing something, we use for and since.
For is used when saying how long it takes to do something (two days, seven weeks etc.).
Since is used when saying a period starts (8 o’clock, Monday).
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