"I'M TALL, but my brother is taller than __" How do you complete this sentence? There are at least three ways. But only one is uncontroversial. The ways in which the other two are irksome illuminate a good bit about English grammar.
The completely correct way to finish the sentence is "than I am." But many times, English-speakers want to truncate what comes after "than". What goes in that slot? Many people think the only correct answer is I: after all, you are shortening I am. But my brother is taller than I feels stilted and stuffy to many perfectly educated native speakers, who would prefer my brother is taller than me. What's going on?