The rule is that, when an adverb is pulled out to the front of a COMPLETE sentence, it becomes a “conjunctive” adverb, begins a brand-new sentence, and needs a period or a semicolon in front of it and a comma after it if it has more than one syllable. It is common to do this with certain adverbs such as
Notice that “so” falls into this category. When it is a conjunctive adverb, “so” means “therefore.”
At the beginning of a complete sentence, these words are conjunctions. They show a relationship between the sentence they start and the sentence that precedes them.
…He submitted his resignation; however, he did not intend to leave at that very moment.
…She was driving northbound at the time; then she turned left.
Sometimes these words are used at the beginning of what is NOT a complete sentence. In these instances, they are no longer conjunctions; they are not connecting anything; they are just adverbs.
…He was taller than everyone else, therefore had no trouble seeing what happened.
…The project began late in the year, so was not completed by the holidays.
The sentence has no conjunction between the two verbs. And since the two verbs cannot back up on each other without something to separate them, we use the comma of omission for that missing conjunction.
…He walked in late; so he was not able to participate. (Connecting word is “so”)
…He walked in late, and so he was not able to participate. (Connecting word is “and”)
…He walked in late, so was not able to participate. (Comma of omission between the two verbs)
…He walked in late and so was not able to participate. (“And” is separating the two verbs)