as / though引导的让步从句必须将表语或状语提前 （形容词, 副词, 分词, 实义动词提前）。
注意：
1) 句首名词不能带任何冠词。
2) 句首是实义动词, 其他助动词放在主语后。如果实义动词有宾语和状语, 随实义动词一起放在主语之前。
Try hard as he will, he never seems able to do the work satisfactorily.
注意:
让步状语从句中，有though，although时，后面的主句不能有but，但是 though 和yet可连用。
only在句首要倒装的情况
Only in this way, can you learn English well.
Only after being asked three times did he come to the meeting.
如果句子为主从复合句，则主句倒装，从句不倒装
Only when he is seriously ill, does he ever stay in bed.
