What is an incomplete verb?
A verb is incomplete if it does not create a complete thought with a subject only.
If we write a sentence like:
The dog barks.
We can see it is a complete thought. Of course we can add some other words to expand the sentence, but those two words are enough to form a complete sentence.
On the other hand if we write:
we can see that this sentence needs more information to be complete. A complete sentence could be:
John seems a nice person.
Verbs which do not form a full sentence with only a subject (incomplete verbs) are: like, seem, be, look, become, etc.