Is So a Preposition or Conjunction?

So can be a conjunction, an adjective, an interjection, a noun, an abbreviation or an adverb.

Is so a preposition?

(So is a preposition complemented by that + clause.)

Is so a conjunction?

So is one of seven coordinating conjunctions represented by the mnemonic FANBOYS: for, and, nor, but, or, yet, and so. When these coordinating conjunctions connect two independent clauses, the conjunction is always preceded by a comma. The grocery store was out of tomatoes, so I borrowed some from my neighbor.