I don’t see it as often as other (as my friend Sarah Townsend calls them) “confusables”, but “since” and “for” sometimes are used interchangeably. Except they aren’t interchangeable.
For
When speaking of time, we use “for” when referencing a span of time.
- I have lived in this house for 10 years.
- I have been married for 21 years.
- I have lived in Lethbridge for 18 years.
- I have been eating supper for 33 minutes.
“For” implies that the thing that had been occurring during this duration will continue into the future. “I have lived in this house for 10 years” implies that the author still lives in the house and that they will live in the house for the foreseeable future.
Since
“Since”, on the other hand, refers to a starting point of time.
- I have lived in this house since 2006.
- I have been married since 1995.
- I have lived in Lethbridge since 1998.
- I have been eating supper since 5:57.
When we use “since”, we specifically refer to a span of time from a specific time in the past up until the present moment.
One final note: “for” can be used in the simple past, as in “She went to Australia for two years.” Since can be used only in the past perfect (“I have been in Australia since 2014.”).