Than tempts writers to use it as a preposition, like but, in such a sentence as "he is older than me". Examples can be found in good writers. But the compilers of the "Oxford English Dictionary" will not have it. We must say "he is older than I" (i.e. than I am). We may say "I know more about her than him" if what we mean is that my knowledge of her is greater than my knowledge of him, but if we mean that my knowledge of her is greater than his knowledge of her, we must say "I know more about her than he(does) ".
But one exception is recognised — whom. We must say "than whom", and not "than who", even though the only way of making grammatical sense of it is to regard than as a preposition. But that is rather a stilted way of writing, and can best be left to poetry:
Beelzebub . . . than whom, Satan except, none higher sat.
Be careful not to slip into using than with words that take a different construction. Other and else are the only words besides comparatives that take than. Than is sometimes mistakenly used with such words as preferable and different, and sometimes in place of as:
Nearly twice as many people die under twenty in France than in Great Britain, chiefly of tuberculosis.