grammaticality - Is it wrong to say "You are smarter than me"?

You are smarter than me.

You are smarter than I am.

Is there any difference between these two? One of my friends told me first sentence is wrong, but the second one is correct. But he could not explain me the reason. So is it true? Iff so, why?

Answer

This used to be a contentious point in the world of English usage. But it's been decided in favor of agreeing that both are acceptable and idiomatic. "Than" can be used as a conjunction or a preposition. When used as a preposition, than takes an object, which can be a pronoun. If the pronoun is first person, it can be "me" because that's what most native speakers actually write and say. It used to be the case that educated native speakers would say "You are smarter than I" and just elide (delete) the "am". But those were the days when some people used "whom", as in "Whom did you give it to?" or "To whom did you give it?" Now almost everyone says "Who'd you give it to?" Almost nobody besides my 89-year-old stepmother (former English teacher) says "You are smarter than I", but some people do say "You are smarter than I am" (than is a conjunction here: "You are smarter than I am smart" is alleged to be the actual substructure of the sentence).

Your friend is merely repeating what the 18th- and 19th-century prescriptive grammarians of the Lowth school laid down as the rules of grammar: that is, rules that told others to speak and write as they (the prescriptivists) thought they should. We ignore those guys now. They lied. And people still have headaches about what proper grammar is and worry about inessentials: specious rules instead of clarity, brevity, and ease of understanding, the hallmarks of good English (but not necessarily of goodness in other languages).