(a) An Adverb of Time
(b) A Conjunction of Time
(c) A Preposition of Time
(a) As an Adverb of Time, it has two meanings.
(i) From then (from some time in the past up to now).
1. I first read it ten years ago and have remembered it ever since.
2. It has pained me in frosty weather ever since.
SINCE with this meaning qualifies a Verb in the Present Perfect and sometimes, as in Indirect Speech, a Verb in the Past Perfect Tense. It is often strengthened with the Adverb EVER and usually follows the Verb it qualifies.
(ii) At or during some time between then and now.
1. He told me last week and has spoken of it several times since.
2. He told me that he had never done so since.
3. I was at school with him but have seen him only twice since.
Here again SINCE usually follows a Verb in the Present Perfect or the Past Perfect Tense.
(b) As a Conjunction it signifies from which time and its use is limited by three conditions.
(1) It is followed by a Verb in the Past Simple Tense.
(2) It is preceded by a Verb in the Present Simple or Present Perfect Tense.
(3) It is preceded by a Noun or phrase denoting some period of time, never by a Noun denoting a point of time.
1. It is now a week since the school broke up. (Correct)
1. Two years passed since my father died. (Incorrect)
2. Two years have passed since my father died. (Correct)
1. It was a week since the holidays began. (Incorrect)
2. It is a week since the holidays began. (Correct)
1. A month has passed since I am coming here. (Incorrect)
2. A month has passed since I came here. (Correct)
1. Two hours have elapsed since he had fallen asleep. (Incorrect)
2. Two hours have elapsed since he fell asleep. (Correct)
SINCE is also a Conjunction of cause or reason.
Since you have finished your work, you may go.
(c) As a Preposition it signifies from and its use is limited by two conditions.
(1) It is placed before a Noun or phrase denoting some point of time, never before a
Noun or phrase denoting a period of time.
(2) It is preceded by a Verb in the Present or Past Perfect Tense.
1. He said that the school had been closed since last Monday.
2. It has been raining since yesterday.
3. Since last month the weather has improved.
4. My father died since last Thursday. (Incorrect)
5. My father has been dead since last Thursday. (Correct)
1. The school was closed since yesterday. (Incorrect)
2. The school has been closed since yesterday. (Correct)
1. My father has been ill since three weeks. (Incorrect)
2. My father has been ill for the last three weeks. (Correct)
1. The results have been known since two days. (Incorrect)
2. The results have been known for the last two days. (Correct)