He won't be here UNLESS he is invited.条件状语从句
He will not go to the cinema BECAUSE he is very busy 原因状语从句
we found the key WHERE she had left it.地点状语从句
we found the book two days AFTER they had gone away 时间状语从句
we had no sooner got to the station THAN the train left 时间状语从句
He speaks english AS IF / AS THOUGH he were an Englishman 虚拟语气
He is explaining clearly SO THAT they could understand 目的状语从句
Do not leave the room UNTIL / UNLESS you have finished the test 时间 / 条件状语从句(这一题出得不严谨)
She sang AS she went along 时间状语从句那第一个为什么不用if a ?如果用 if ,句子就要改写成：He will be here IF he is invited.==He won't be here UNLESS he is invited.