since:

          since从句中的动词不同，其起算时间也不同。" since+瞬间动词过去时 " 是从该动作发生时起算；" since+延续性动词过去时 " 是从该动作结束时起算。试比较:

　　He has worked very hard since he entered the factory.

　　( enter为瞬间性动词，该句为: 从他入厂以来，他工作一直非常努力。)

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

　　( work为延续性动词，该句为: 自从我不在工厂以来，我们没见过面。可说成: We haven't wen each other since I stopped working in the factory.)

　　since常用于“It' a+some time+since sb. did sth“结构。试比较:

　　It's two years since he joined the army.他参军有两年了。

　　It' a two years since be was a soldier.他退役有两年了。

before:　　

        before有时可译为“还没……就“。

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

                     我还没来得及说话, 他就给我量好了尺寸。

　         　I hadn't waited long before he came.

                     我还没等多久他就来了。

　　         I slipped out before the lecture started.

                     还没等讲座开始我就溜了出去。

　　 before也可用于it结构，与 since 结构比较相似，注意它们所用时态的区别，其结构通常是:"It will be + some time + before sb. does sth.’或" It was + some time + before sb. did sth. "。可表示将来或过去两个概念。常译作“……才……”。

　　【例】 

      He is leaving for Australia and it will be three years before he comes back.

                      他要去澳大利亚了，3年后才会回来。

　　Duing the war he joined the army and it was three years before he came back.

                      战争期间他参了军,3年后才回来。