conjunction (used with the present perfect and the past perfect tenses)
1 at a time after a particular time or event in the past: In the 12 months since I last wrote to you a lot has happened to me. | I can't have seen him since 1983. | It's been years since I enjoyed myself so much.2 during the period of time after a particular time or event in the past: Since he started that diet he's lost over 20 lbs in weight. | ever since: We've been friends ever since we met at school.3 used to give the reason for something: I'll be forty next month, since you ask. | Since you are unable to answer perhaps we should ask someone else.2 preposition (used with the present perfect and the past perfect tenses)1 at a time in the past after a particular time or event: They haven't met since the wedding last year. | Since the end of the war over a dozen hostages have been released.—compare for 1 (8)2 for the whole of a long period of time after a particular time or event in the past: Since the day we met I have known he was not to be trusted. | ever since: Ever since the war she's been able to feed a whole family with a few potatoes and eggs.3 since when? spoken used in questions to show surprise, anger etc: Have you checked this bill? Since when does -42 plus -5 service charge come to -48?3 adverb (used with the present perfect and the past perfect tenses)1 at a time in the past after a particular time or event: He husband died over ten years ago but she has since remarried. | I've since forgotten what our argument was about. | He walked out of that door last Tuesday and no one's seen him since.2 for the whole of a long period of time after a particular time or event in the past: The accident happened four years ago and she has hardly spoken since. | ever since: We came to the UK in 1974 and have lived here ever since.3 long since if something has long since happened, it happened a long time ago: I've long since forgiven her for what she did.USAGE NOTE: SINCE WORD CHOICE: since (prep/conj), from, after, from...to/till/until, for Since is mainly used where you want to talk about a state or activity that started at some time in the past and has continued to the time when you are speaking: I've been here since ten o'clock this morning. | The place had completely changed since I went there three years ago (NOT It has changed since three years/ three years before). From or after may be used to show the starting points of periods of time where you do not use since. For example: I hope they'll be friends from now on (NOT since now on), means I hope they will be friends from now and into the future. She was very unhappy for a while after leaving home (NOT since) means that she was unhappy from a period of time in the past until a later time in the past. From...to/until/till is used where you want to give both ends of a period of time during which some state existed or some activity was being done. This construction can go with most tenses of the verb: I was here from ten till two. | From 1990 to the present he's had no regular job (NOT since 1990 to the present).). | She works from sunrise until sunset. For is used where you want to give the length of a period of time, but do not need to say exactly when it started or finished. It goes with all tenses of verbs: We lived there for a long time. | She's only staying for a week. When you use for with the present perfect tense, it gives a period of time that ends at the time of speaking: I've been waiting for two hours (NOT since two hours). In spoken English the for is often left out: I've been here two hours. | She's only staying a week. GRAMMAR The point of time with since may be shown by a clause, which may contain a verb in the simple past: He's been ill ever since he arrived. The point of time with since may also be shown less exactly, by mentioning a period of time that ended in the past: He's been working here since last week/the 60s (= he started at some time during the 60s). | Since I was a kid I've wanted to visit Disney World. A since clause may also itself cover the whole period from a point in the past to the time of speaking: Since she's been living here she's made a lot of friends. However, as in all the above examples, the main verb in any clause with since usually has to be in one of the perfect tenses. Compare also: Yesterday Bobby told me he hadn't eaten since Tuesday (= between Tuesday and yesterday he did not eat anything). Non-perfect tenses are used only in particular situations, for example where you are talking about the length of time itself: It's two weeks since I've seen you (NOT ...since I haven't seen you). | It seems like months since you last paid me. Note also: Since the car accident she can't walk properly (= she hasn't been able to walk properly).
Longman dictionary of contemporary English. 2004.