Do not force you pedals with a wrench if they will not thread in by hand.

Please please please please please do not force them.

The threads on the crank are weaker than those on the pedal, and are very easy to strip.

You can also try Simple Green + a brush with plastic bristles (like the Park cassette cleaner) on the threads, then grease everything, and try to insert again. You do have to get the angle exactly right, or they won't go in.