英语问题，如何区别not...unless...,not...until...和not...before...

until he has come 强调直到....才 是一种临界状态 直到他回来我才完成作业 before he came 强调再他回来之前 我没法完成作业 一、作为从属连词含义不同,引导从句性质不同 unless与until均为从属连词,unless表“除非”,强调条件,引导条件状语从句;until表“直到”,强调时间的延续,引导时间状语从句。 1. I didn't know the news until my teacher told it to me.