You need to conjugate verbs in most languages. Whether you
realize it or not, you conjugate verbs in English as well as in
French. Use the French verb "avoir" for example. "j'ai" translates
to "I have", and "il a" translates to "he has". If you didn't
conjugate it and just left it as "j'avoir" then that would
translate to "I to have" which is obviously incorrect.