I got the wrong answer when playing the 'JLPT Taisen' game. The question is: "田中さんとは、学校を_____以来、会っていない". I think the answer is "卒業した", but "卒業して" is correct.
Please teach me why?
「~て以来」 is a set phrase. You just use it like that. It means "since + (an action described by a verbal sentence)".
「~た以来」does not have any sense. It is not possible to precede 以来 with a verb in the ~た form.