Are there simple past and simple future verb exercises in Duolingo for French learners?
Duolingo doesn't teach the passé simple and has a small lesson on the futur simple (near the bottom of the tree). The site given by MikeNolan6 is very good but also does not drill on passé simple (however, it does discuss them). See here: https://www.laits.utexas.edu/tex/gr/tap10.html
EDIT: I just remembered this course on Memrise is terrific for learning conjugations - https://www.memrise.com/course/756657/verb-conjugations-en-francais/.