He doesn't SEEM (to be a nice person) and for the other one.. That dog eats ;)

Only the first verb takes an ending. Example: Did he work? ("Did" is in the past, "work" is the base form). He worked a lot ("work is in the past). It is possible to say "that dog does eat" -- the use of "do" (or "does" or "did") in an affirmative statement is emphatic, like saying "that dog really eats." Only the helping verb takes an ending.