Several weeks ago, Bob Scopatz asked in a comment about the word however, specifically whether it should be preceded by a comma or a semicolon when it’s used between two clauses. He says that a comma always seems fine to him, but apparently this causes people to look askance at him.
The rule here is pretty straightforward, and Purdue’s Online Writing Lab has a nice explanation. Independent clauses joined by coordinating conjunctions are separated by a comma; independent clauses that are not joined by coordinating conjunctions or are joined by what OWL calls “conjunctive adverbs” require a semicolon.
I’ve also seen the terms “transitional adverb” and “transitional phrase,” though the latter usually refers to multiword constructions like as a result, for example, and so on. These terms are probably more accurate since (I believe) words and phrases like however are not, strictly speaking, conjunctions. Though they do show a relationship between two clauses, that relationship is more semantic or rhetorical than grammatical.
Since however falls into this group, it should be preceded by a semicolon, though it can also start a new sentence. Grammar-Monster.com has some nice illustrative examples:
I am leaving on Tuesday, however, I will be back on Wednesday to collect my wages.
I am leaving on Tuesday; however, I will be back on Wednesday to collect my wages.
I am leaving on Tuesday. However, I will be back on Wednesday to collect my wages.
The first example is incorrect, while the latter two are correct. Note that “however” is also followed by a comma. (But would also work here, though in that case it would be preceded by a comma and not followed by one.)
Bob also mentioned that he sometimes starts a sentence with “however,” and this usage is a little more controversial. Strunk & White and others forbade however in sentence- or clause-initial position, sometimes with the argument that in this position it can only mean “in whatever way” or “to whatever extent.”
It’s true that however is sometimes used this way, as in “However it is defined, the middle class is standing on shaky ground,” to borrow an example from COCA. But this is clearly different from the Grammar-Monster sentences above. In those, the punctuation—namely the comma after “however”—indicates that this is not the “in whatever way” however, but rather the “on the contrary” or “in spite of that” one.
Some editors fastidiously move sentence-initial “howevers” to a position later in the sentence, as in I will be back on Wednesday, however, to collect my wages. As long as it’s punctuated correctly, it’s fine in either location, so there’s no need to move it. But note that when it occurs in the middle of a clause, it’s surrounded by commas.
It’s possible that sentence-initial however could be ambiguous without the following comma, but even then the confusion is likely to be momentary. I don’t see this as a compelling reason to avoid sentence-initial however, though I do believe it’s important to punctuate it properly, with both a preceding semicolon or period and a following comma, to avoid tripping up the reader.
In a nutshell, however is an adverb, not a true conjunction, so it can’t join two independent clauses with just a comma. You can either join those clauses with a semicolon or separate them with a period. But either way, however should be set off by commas. When it’s in the middle of a clause, the commas go on both sides; when it’s at the beginning of a clause, it just needs a following comma. Hopefully this will help Bob (and others) stop getting those funny looks.