Since as a preposition

In sentences with since, we normally use present perfect and past perfect tenses in the main clauses.

I have known her since 1990. (NOT I know her since 1990.)
It has been raining since yesterday.
We have lived in this city since 1995.
We haven’t seen him since Christmas.

I was upset when Alice decided to move to America. We had been good friends since our school days.

In sentences about changes, present and past tenses can also be used.

You are looking much better since your operation.

Note that this usually happens in the structure It is/was … since.

It is a long time since our last meeting. OR It has been a long time since our last meeting.

Since as a conjunction

In the examples given above, since was used as a preposition. Since can also be used a conjunction of time. The tense in the since-clause can be perfect or past.

I have known her since we were kids.
I have known her since I have come to this city.

Since and for

Both since and for can be used as prepositions with similar meanings. Since is used when we are reckoning from a particular date or point of time. Examples are: since last Friday, since yesterday, since 2 o’clock, since morning etc.

For is used for a period of time. Examples are: for a week, for a long time, for two hours etc.

Compare:

I have been waiting since morning.
I have been waiting for ages.
He has been ill for three months.
He has been ill since July.