Grammatical myths arise (“Never end a sentence with a preposition.”) and are often difficult to dispel. One such “rule” is that sentences should never begin with however. Not so.
An illustration: “The Phillies were on the verge of a big inning with the bases loaded. However, the relief pitcher doused the fire.” Or: “Congress was determined to increase the tax on gasoline. However, the President’s veto was sustained.”
In both of these illustrations however is followed by a comma because contrast is intended. But might be used in either sentence, instead of however, as its meaning would be the same; the only reason it might be preferable would be its brevity. Consider also the use of yet instead of however, though its meaning is a shade different.
However is not followed by a comma when it is used to mean “in whatever way” (“However they managed through a blinding snowstorm, the climbers reached the pinnacle of the mountain.”) or “to whatever extent” (“However the efforts of the border patrol to discourage them, the refugees continued to surge into their new country.”), and its use in either of those illustrations is unimpeachable, according to “Garner’s Modern American Usage.”
Used other than at the beginning of a sentence, however puts emphasis on the word that precedes it. “They all left at the same time. John, however, arrived late because of traffic.” In the illustration John is distinguished from the others, who were not delayed by traffic, by however.
Where however is often misused is in run-on sentences. “The car went through the intersection on a red light, however, the video camera captured the event.” In that sentence however is used improperly as a conjunction. It is an adverb, and since both of the clauses is a complete sentence, the comma after “red light” should not have been used. Either a semi-colon or a period after “red light” is correct.
– Ken Butera