Since usage.

Since is an extremely active word and a key word used in the college entrance examination. Its usage is summarized as follows:
1. Used as a prefix, which means "from ...... Since, ", often used with completion. 　　

I haven't written home since Christmas.
I have never written a letter back home since Christmas.
　　
2. Used as an adverbs, meaning "from then on (till now)" and "after (between now )"; since then, ever since has been (more emphasis on since usage ).

　　
Things have not changed very much since.
Since then, the situation has not changed much.
His first book was a great success and he has been famous as a writer ever since.

His first book was very popular and he has become a writer since then.
He got a job with the firm in 1970 and has worked there ever since.
He has been working in that company since 1970 and has been working there since then.
　　
Iii. Used as a conjunction
　　
　1. the adverbial clause of the guiding reason, which is equivalent to now that, which means "because ......, Since ......, In view ......".

Since the rain has stopped, let's go for a walk.
Now that the rain stops, let's take a walk.
Since we are young, we shouldn't be too afraid of making mistakes.
Because we are still young, we should not be too afraid to make mistakes.
Note: When since indicates the cause, the tone is weaker than because. It indicates that the cause is known by the other party or is not very important in the sentence, therefore, the guiding clause is not the focus of information. It cannot answer why's questions or be used as an emphasized content in the stressed sentence pattern. The clauses guided by since are usually placed before the main sentence.

　　2. the predicate clause of the guiding time is complex.

　　(1) If the predicate verb of the adverbial clause guided by since is the past tense of a final verb (such as go, come, leave, start, and begin, then the clause indicates the time starting from the moment of the action ". This is the most common usage.For example:

He has studied very hard since he came to our school.
He has worked very hard since he came to our school.
Great changes have taken place in our school since you left.
Since you left, our school has changed a lot.
　　(2) If the predicate verb of the adverbial clause guided by since is the past tense of a continuous verb or a static verb indicating a State (such as live, stay, study, learn, smoke, and be, the time indicated by the clause is counted from the completion or end time of the action or state ".For example:

I haven't heard from him since he lived here.
I have never received his letter since he does not live here.
I haven't eaten snkes since I was a student at school.
Was indicates the status, so this sentence can be understood as: I haven'teaten snkes since I left school.
I have never eaten snakes since I graduated.
Comparison:
He has written to me frequently since I was ill.
He has frequently written to me since I became ill. (The clause is a state verb)
He has written to me frequently since I fell ill.
He often writes to me since I was ill. (The clause is a final verb)
　(3) If the predicate of the adverbial clause guided by since is the current completion of a continuous or static verb, it indicates that the action and State continue to the present (speaking moment ), the start point of the time is counted from the time when the action occurs.For example:

He has written to me frequently since I have been ill.
Here, have been ill indicates the continuity of the State. The starting point of time should be counted from the time when you start to get sick. Therefore, he often writes to me since I was ill ."
Comparison:
I 've written her 20 letters since I have been here.
Since I came here, I have written 20 emails to her. (Starts from)
I 've written her 20 letters since I was here.
Since I left, I have written 20 letters to her. (From the end)
　(4) It is + time + since.../It Has Been + time + since ...... How long has it been ". The two sentences share the same meaning. The former is a standard statement, and the latter is a verbal statement.

Usage: common past tense is often used in the since clause. If the clause is the general past tense of a continuous verb, it indicates how long the action has ended. For example:
It is three years since he smoked.
He has not smoked for three years.
Comparison:
It is three years since he began to smoke.
It is three years since he has smoked. He has been smoking for three years.
Note: In this structure, the clauses guided by since cannot use negative predicates to represent negative meanings. Try to compare them:
He has not learned Chinese for a long time.
It is a long time since be didn't study Chinese.
It's a long time since he studied Chinese.
(Studied is a continuous verb and should be counted from the end of its action)
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