Than (conjunction) + clause
Incorrect : He is taller than me.
Correct : He is taller than I (am).
Note: The word ‘than’ is a conjunction and hence should be followed by a clause beginning with the subject form of a pronoun. The verb after the pronoun is generally omitted for the sake of brevity.
The first ‘Incorrect’ form is often used in spoken English. It is now acceptable English. In that case, ‘than’ is regarded as a preposition.