在某些副词之后使用逗号: however, in fact, therefore, nevertheless, moreover, furthermore, still, instead, too (含义'also').

Example: Therefore, he didn't say a word.

如果这些副词出现在句子的中间，则用逗号括起来。

Example: The thief, however, was very clever.

逗号对于以下副词是可选的: then, so, yet.

Example:
 So, she entered the house.
 So she entered the house.

therefore [´ðeəfɔ:]   ad.&conj.因此；所以   (初中英语单词)