For and since both relate to time, but they do different jobs.
Use for to talk about a length of time. It answers the question how long. For example, I studied for three hours. The phrase three hours is a duration.
Use since to talk about the starting point of an action or situation. It answers the question since when. For example, They have worked here since Monday. Monday is the point when the action began.
A helpful pattern is this: for + period, since + point in time.
- for: for ten minutes, for a week, for a long time
- since: since 9 o’clock, since April, since last year
Since often appears with the present perfect when something started in the past and still continues now. Example: She has known him since childhood.
If you remember duration versus starting point, choosing the right word becomes much easier.