People are familiar with using a comma before and, but, or so, but however, nevertheless, and furthermore are a different kind of connecting word and can be more problematic.
Nearly three dozen words that appear at the start of a sentence and in the middle serve a “connecting” function by showing the relationship between thoughts. They express such things as addition (besides, also), emphasis (certainly, in fact), comparison (nonetheless, similarly), cause/effect (consequently, as a result), and time (meanwhile, then).
Two characteristics make these words different from other conjunctions:
They usually connect complete clauses, not simply a few words, so when used in mid sentence, they require a semicolon before them and a comma after them. Here are two examples:
- Several people were on vacation; consequently, we missed the deadline.
- We have plenty of time; however, you might want to finish early.
What can be more difficult is determining when to use commas around them in mid sentence:
- Yes: It’s a great idea. We don’t, however, have the budget. (However is used in contrast to what was said in the previous sentence.)
- No: The deadline is Friday. We therefore need to work overtime to finish. (Therefore is essential to the main thought of the sentence, so it is not set off with commas.)
Use your voice as a guide. Read the sentence aloud. If your voice rises as it pronounces the adverb, that tells you the word is essential to the main thought and therefore not marked off by commas.