4) 比较状语从句
　　比较状语从句主要由以下连词引导：(not) as … as, (not) the same as, (not) such … as, not so … as, than, the + 比较级…, the +比较级…等。
　　We have had a lot more rainfall this year than we had last year.
　　She can speak English as fluently as her teacher (can).