在並列句和狀語從句這個專題中，考點大概有三個：一、狀語從句的引導詞與介詞的用法比較;狀語從句與定語從句引導詞的辨析。二、同類狀語從句中近似連線詞的用法差異。三、whatever、whenever、wherever等引導詞與no matter wh-等引導詞的不同用法。在這裡，我們就先來看看由before與since引導的時間狀語從句有何區別。

before與since引導的時間狀語從句：

1、如果表達“還未……就……;不到……就……;……才……;還沒來得及……就……”時，需用連詞before.

例句：We hadn't run a mile before my sister felt tried.

我們跑了還不到一英里我妹妹就感到累了。

2、before從句中謂語不用否定式。

Before they reached the station, the train had gone.

他們還沒有到火車站，火車就已經開走了。

3、It will be + 一段時間+before…還要多久才……。

例句：It will be a year before I come back.

還得再過一年我才能回來。

4、since 從句的謂語動詞一般是非延續性動詞，主句的謂語動詞是延續性的或者是反覆發生過的動作。since從句的時態若是一般過去時，相應地，主句中的時態是現在完成時或現在完成進行時。

例句：I have written home three times since I came here.

自從我來到這兒，我已經給家裡寫過三封信了。

5、在“It is + 一段時間 +since 從句”句型中，其句意取決於從句的謂語動詞是否可延續。

例句：It is two years since the war broke out.

自戰爭爆發以來有兩年了

since從句中的動詞不同，其起算時間也不同。' since+瞬間動詞過去時 ' 是從該動作發生時起算；' since+延續性動詞過去時 ' 是從該動作結束時起算。試比較:

　　He has worked very hard since he entered the factory.

　　We haven't seen each other since I worked in the factory.

　since常用於“It' a+some time+since sb. did sth“結構。試比較:

　　It's two years since he joined the army.他參軍有兩年了。

　　It' a two years since be was a soldier.他退役有兩年了。

before:　　

before有時可譯為“還沒……就“。

　　【例】He had measured me beforer I could say a word.

我還沒來得及說話, 他就給我量好了尺寸。

　 　I hadn't waited long before he came.

我還沒等多久他就來了。

before也可用於it結構，與 since 結構比較相似，注意它們所用時態的區別，其結構通常是:'It will be + some time +before sb. does sth.’或' It was + some time + before sb. did sth. '。可表示將來或過去兩個概念。常譯作“……才……”。

1.Close the door before you leave the room. 離開房間前關上門。

2.she worked in shsnghai before she came here.she had studied english for six years before she went to college.

3.It has been almost five years since I taught high school students.我不教高中已經大概有五年了。