1、 however主要有两种用法：
1）however = no matter how，做连词，用于引导让步状语从句
如：He can't win the race however fast he runs.
无论/不管他跑得有多快，他都赢不了这次赛跑。
2）however做副词，表示转折，意思是“然而/但是”，用在句中时必须用标点符号和其他成分隔开。
如：He is clever. However， he is very lazy.
2、 therefore只能做副词，意思是“因此/所以”，其所在的句子表示结果。
它和句子其他成分隔开不隔开都是可以的。
如：He worked day and night， and therefore he was able to buy the sports car.
= He worked day and night. Therefore， he was able to buy the sports car.
他日夜工作，所以有能力买那部跑车。
