a) Piel usually expresses an "intensive" or "intentional" action.
he broke he broke to pieces, he smashed
he sent he sent away, he expelled
b) Sometimes the Piel introduces a new meaning to the Qal form.
he counted he recounted, he told
he completed he paid, he compensated
he learned he taught
c) Piel expresses a "repeated" or "extended" action.
he jumped he skipped, he hopped
d) Some intransitive verbs in Qal become transitive in Piel.
to be strong to strengthen, to fortify
to become great to make great