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for
vs
since

The prepositions for and since are often used with time expressions.

For indicates a period of time.

For example:

I have been working here for 2 years.

Since indicates a point in time.

For example:

I have been working here since the year before last.

!Note - Just remember this sentence "For a length of time, since a point in time."

See the grammar pages for more information on prepositions of time.