I’ve known Theo _______ 5 years. Is it for or since? Easy:
- I’ve known him for a long time, it seems.
- I’ve known him since he showed me his collection of old records and we discovered that we share a hobby.
- Incorrect: I know him for a long time -> I’ve known him for a long time.
Compare my chart for “until/ by”. I developed these charts a few years ago and get a lot of mileage out of them.