For or since?

We use for with a period of time in the past, present or future.

We use since with a point in time in the past.

Compare

He lived in Oxford for 3 years and then he moved. (completed past)

They’ve lived in Oxford for a couple of months. (began in the past and continues into the present)

We’re going to stay in Oxford for three days and then we’re going to London for a day. (future plans)

For refers to periods of time, e.g. 3 years, 4 hours, ages, a long time, months, years.

They’ve lived in Oxford since 2004.

Since refers to a previous point in time.

Warning:

We don’t use since with periods of time:

She’s been on the phone for hours.

Not: She’s been on the phone since hours.