The part of speech of "than"

In this sentence "He did more than tell you the right preposition to use."
why it is wrong to replace " tell " with "told" or " telling"?
What's the part of speech of "than" in the sentence?
if it is a conjunction, the verb "tell" should coordinate with the past tense of "do" and thus changed into " told".
If it is a preposition , "tell"should be changed into a "gerund" "telling".
can you explain this to me ? Thanks!