�、�. 三者都能用來表示時(shí)間，但用法不同。
since “自從”，所表示的是一個(gè)時(shí)間點(diǎn)。可用作介詞，也可用作連詞，后接時(shí)間名詞或短語，或引導(dǎo)時(shí)間狀語從句;要求前面的謂語動(dòng)詞或主句的謂語動(dòng)詞必須用完成時(shí)態(tài)，要用延續(xù)性動(dòng)詞。而since引導(dǎo)的從句中的謂語動(dòng)詞通常應(yīng)是短暫性的動(dòng)詞。若接時(shí)間，則應(yīng)為點(diǎn)時(shí)間。如：
① He has worked there since1989.
�、� She has lived here since she moved here.
Ⅱ. from “自從” 只用作介詞，表一個(gè)事情的開始點(diǎn)，可用于過去、現(xiàn)在或?qū)淼臅r(shí)態(tài)。如：
�、� They worked from 7:00 to 12:00 this morning.
�、� We have been good friends from childhood.
Ⅲ. for 作為介詞，后面接段時(shí)間，用于完成時(shí)、現(xiàn)在時(shí)、過去時(shí)和將來時(shí)，句中要用延續(xù)性動(dòng)詞。如：
① We’ll stay here for ten minutes.我們將在這兒呆10分鐘。
② They have studied English for three yeas.
，我們將會及時(shí)處理。