Commas with Adverbs - Writing English
Use a comma after certain adverbs: however, in fact, therefore, nevertheless, moreover, furthermore, still, instead, too (meaning 'also').
Example: Therefore, he didn't say a word.
If these adverbs appear in the middle of a sentence, they are enclosed in commas.
Example: The thief, however, was very clever.
The comma is optional for the following adverbs: then, so, yet.
So, she entered the house.
So she entered the house.
see also: → Form of Adverbs