Sometimes an adverb gets pulled out to the beginning of a sentence to form a “bridge” to the sentence before it. It becomes a linking word for the two sentences and shows a relationship between the two sentences. This is called a conjunctive adverb. Some examples are
however, moreover, nevertheless, therefore, still, thus, yet, then
Generally there is a comma before a conjunctive adverb. When it has more than one syllable, it has a comma after it; when there is only syllable, there is no comma.
…He had sent her the money. He therefore expected a response.
…He had sent her the money; therefore, he expected a response.
…I had explained it several times. He still didn’t understand.
…I had explained it several times; still he didn’t understand.