Since is used for time and means "from that time to the time referred to." It is often used with a Present Perfect or Past Perfect Tense:
- She has been here since morning. (from morning till now)
- She wondered where Tom was. She has not seen him since they graduated from college.
Since can also be a conjunction of time:
- He has worked for us ever since he dropped out of school.
- It is ten years since I last saw Garry. (I last saw Garry ten years ago/I haven't seen Garry for ten years.)
For is used with a period of time: for three days, for two years, for ever:
- Cook it for one hour.
- They were camping in the mountains for two weeks.
For + a period of time can be used with a Present Perfect Tense or Past Perfect Tense to show an action that started some time ago and lasted up to the time of speaking:
- They have lived in this house for five years. (They started living here five years ago, and they still live here.)
For used in this way can be replaced by since with the point in time when the action began.
- They have lived here since this time five years ago.