These words are often confused in writing and sometimes in pronunciation. Than is a conjunction in clauses of comparison: "He worked better today than he did yesterday." Then is an adverb of time: "We then went to a restaurant." Think of than only as a conjunction; it will then be easy to remember that a following pronoun should have the same case as its antecedent. Say "Everyone knows more about the situation than he." Say "The supervisors counted on no one more than him." "Someone then remarked: 'It is better to remain silent and have some people think me stupid than to say something and remove all possible doubt. '"
Dictionary of problem words and expressions. Harry Shaw. 1975.