A. For refers to a space of time;
since and from refer to a point of time ;
but, while since refers only to a past point, from refers to a point of time in all tenses :
He has been here for a week. I have not seen you for many days. It is many days since I saw you last (since the point of time that I saw you last). He has been here since Monday last. He began English from the age of ten (past). He begins English from today (present). He will begin English from tomorrow (future).
As adverbs, since and ago often mean the same thing. Thus, write either "I saw him long ago or long since."
Note that long since - long ago (not, for a long time).
If 'Ago' is used and the event to be dated is by a clause, it should not be followed by the conjunction 'since' but should have the conjunction that. It was ten years ago that (not since) he died. The alter native constructions are : It is ten years since he died. He died ten years ago. Ago since is tautological, because adverbially since ago.
NOTE : Carefully guard against the use of ago and since together:
It is five years ago since he came here. (Incorrect).
He last came here five years ago. (Correct). It is five years since he came here.(Correct).
B. The use of since' in the different parts
(i) In an Adverb, it has two meanings:
(a) From some past time up to now:
I met him three years ago and have known him ever since.
He said that he had been healthy since.
The price was high last year; but it has come down since.
I was with him at school; but have seen him only thrice since.
In this sense, since follows a verb in the Present Perfect or Past Perfect Tense.
(b) Ago:
I saw him two months since. He died long since
In this sense since follows a verb in the Past Indefinite Tense.
Modern writers prefer 'ago' to 'since' in this sense.
(ii) As a Preposition, it means from, and is placed before a noun or a phrase denoting some past point of time, and is preceded by a verb in the Perfect Tense:
I have been since yesterday. He has been here since January last. They have been here since the 5th instant.
(ii) As a Conjunction, it means from the time when, and is followed by a clause with a verb in the Past Indefinite and preceded by a verb in the Present Indefinite or Present Perfect:
I have been ill since you saw me last. It is a month since I saw you last. They have been absent since you left the place.
