sentence adverbs
An adverb that applies to the statement as a whole, not to a verb.
A sentence adverb applies to the statement as a whole. Words such as clearly, fortunately, frankly, and obviously are standard in that role. Set it off. Use one comma when it opens or closes the sentence. Use a comma on each side when it stands in the middle. An adverb that modifies a verb takes no comma.
Examples
UseFrankly, the invoice was late. (opens the sentence)
AvoidFrankly the invoice was late. (not set off; nor does it modify a verb)
UseThe invoice, frankly, was late. (stands in the middle)
UseThe invoice was late, frankly. (closes the sentence)
UseHe answered frankly. (modifies a verb, so no comma)