Phrasal verbs are mostly nonsensical.
Which, together with the huge range of them, must make them a nightmare for non-native learners. You can
fill in or
fill out a form but probably not
fill up one or just
fill one; whereas you can just
print a document if you're being economical, rather than going to the effort of printing it
out or
off.
Hitting on an idea and
hitting on a person are quite different, and if you
hit on someone you may be lucky enough to
hit it off with him/her as well. Etc etc. How do foreigners manage?