"However" is the most common "conjunctive adverb." Conjunctive adverbs are a large group of words that connect two sentences.
Conjunctions are words like "and," "or", and "but" that connect two halves of a sentence across a comma ( , ).
"I like chocolate, *but* Sam likes vanilla."
Conjunctive adverbs like "however," "nevertheless," and "furthermore" connect two sentences across a semicolon ( ; ), colon ( : ), or period ( . ) You need to use a comma after them.
"I like chocolate. *Nevertheless*, Sam prefers vanilla."
Conjunctive adverbs can also be used to strengthen conjunctions after a comma. You NEED to use a conjunction BEFORE a conjunctive adverb after a comma.
"I told Sam to stop eating vanilla, *but nevertheless* he continued doing it."
You can also use conjunctive adverbs to interrupt a clause. When you do that, you should put commas on both sides.
"Sam continued to eat vanilla. When, *however*, I punched him, he stopped. Sally, *though*, told him that he should continue eating it. She said he shouldn't listen to bullies like me."