When do you use “since” and when “for”?

There is a rule, and it is a clean one. Use **since** with a point in time — the moment something started. Use **for** with a length of time — how long it has lasted. Ask yourself whether the words after the gap name a moment or a duration, and the choice follows.

• Points in time, so “since”: since 2019, since Monday, since April, since I was a child, since we moved here.
• Lengths of time, so “for”: for three years, for two hours, for a long time, for ages, for a week.

A useful check: a number on its own is almost always a duration, so it takes “for” — “for six months”. A year, a date, a day name or a whole clause naming an event is a starting point, so it takes “since” — “since June”, “since she left”. Notice that “since” can be followed by a clause while “for” cannot.

Both usually sit with the present perfect or present perfect continuous, because both describe something that began earlier and still holds: “I have lived here for five years” and “I have lived here since 2021” describe the same situation from the two different angles. One measures the stretch, the other names the start.

One caution: “since” has a second, unrelated job as a conjunction meaning “because” — “Since it was raining, we stayed in.” That has nothing to do with time, and the time rule above does not apply to it. Context makes which one is meant obvious.