Readers often write in to complain about starting a sentence with so, even suggesting that it sounds condescending. Anand Giridharadas of the New York Times agrees that there can be a “logical tinge to so… Compared to well and um, starting a sentence with so uses the whiff of logic to relay authority.” The Telegraph informally confirms this feeling too, claiming that it may sometimes “send the wrong message: It could alienate colleagues who believe they're being spoken down to when they hear it.” One inquirer on the English Language and Usage site asks, “Am I the only one who finds it annoying?”
So Can Be Used in Many Ways
Not all types of so raise objections, so let’s look at some of the different roles this little word can play. When it is not at the beginning of a sentence or clause, it is often used as an adverb that modifies (or intensifies) an adjective, with a meaning similar to adverb very, like this:
(1) Babies are so cute!
This usage is very common in informal speech, but some style guides and editors do complain about this type of so also, as explained in this article about using so as an intensifier.
Next, let’s look at some “uncontroversial” uses of so, and take a closer look at some types that appear at the beginning of a clause. One of the most common and undisputed ways we hear and think of using so is as a coordinating conjunction, like this:
(2) I love grammar, so I do research on it and write about it.
The conjunction so in this compound sentence implies that the first part, “I love grammar,” is the cause of the second part, “I do research on it and write about it.” In addition, it prevents a comma splice. (Read this article for more about comma splices.)
It is also common to see so used for the same purpose but at the beginning of a sentence, meaning that the comma from our last example is replaced with a period like this:
(3) I love grammar. So, I research and write about it.
In that sentence, so is a conjunctive adverb. One way to understand why this one is a type of adverb is to see that it modifies the whole sentence that follows it, as adverbs often do. It is also synonymous with the word therefore and the less-common ergo; these conjunctive adverbs are often called “transitions” in formal writing classes. These types of adverbs at the beginning of a sentence (also sometimes called modifiers) must be followed by a comma. (Even when conjunctive adverbs occur in the middle of a sentence, which some can, they must be offset by commas, like this: “I did finally learn to ride a bike, however, when I was ten years old.”)
Further, even though this so is at the beginning of a sentence, it is not the type that peeves people. All of these conjunctive adverbs are, however, considered what linguists call “discourse markers” (4). (We can think of the word “discourse” as referring to any dialogue or conversation.)