1. as / though 引起让步状语从句，可将表语或状语置于句首，但主谓不颠倒顺序。如：
Tired as / though he is, he is still working.
Carefully as / though she listened, she didn't catch a word.
若将含有不定冠词修饰的表语提前，则应省去不定冠词 a / an 。如：
Child as / though he is, he knows a lot.
2. however, no matter now 修饰形容词或副词时要前置，用以加强语气，该让步状语从句的主谓不倒装。如：
However hard the problem is, I am determined to work it out.
No matter how heavily it was raining outside, he insisted going there.