Then and than are among the 100 most frequently used words in the English language. For some, this ubiquity translates into greater opportunity for committing grammatical blunders. Let’s take a look at the differences between these two terms.
Then indicates time or consequence, as in the following examples: Bagels were cheaper then; First I’ll drink my orange juice, then eat my bagel; If I drink too much orange juice, then I won’t have room for a bagel.
Than is used to indicate comparison: He likes bagels more than I like bagels. However, things get a little trickier when we consider how to abbreviate this sentence. Is it He likes bagels more than I, or He likes bagels more than me? Traditionalists will argue that than is a conjunction, and that the pronoun in the subordinate clause should be in the subjective case (I, he, she, they): He likes bagels more than I. In this construction, the reader is able to effectively and accurately finish the sentence in his or her mind, “more than I like bagels.”
However, in informal communication, than is often treated as a preposition, and the pronouns in the second element are in objective case (me, him, her, them): He likes bagels more than me. Although you’ll often be able to get your point across just fine with than me, be aware that for attentive readers and listeners, it can introduce ambiguity: does he like bagels more than I like bagels? Or does he like bagels more than he likes me?
To avoid confusion, your best bet is to use than I (or than he, than she, than they) in formal and professional settings, and reserve than me (along with than her, than him, than them) for informal speech.