Use “since” when a specific point in time is given. Use “for” when a period of time, rather than a specific point in time, is given.
|last week||a week|
|last month||three months|
- Richard has lived in Chicago since 1985.
- Richard has lived in Chicago for twenty years.
Notice that you can use for and since with either the simple past tense or the present perfect tense, depending on whether or not the action is still happening.
- I’ve worked for Green Enterprises for three years. (I still work there.)
- I worked for Green Enterprises for three years. (Now I work somewhere else.)