I'm a novice student of Greek getting back into the language and have what should be a fairly simple question about accent shift in contract verbs. Basically, my question has to do with how the accent works on the single imperative form of philew - I love. (Sorry for the lack of Greek letters!) I know that verbs' accents are recessive and I've been told that in contact verbs the accent cannot recede beyond the syllable where the contraction takes place.
However, all my resources show that the singular imperative of phileo - philei - has the accent recede to the first syllable, even though all the other active voice forms retain a circumflex on the last syllable. Could anyone please explain why the accent shifts to the penult in only this case?