Is this sentence grammatical?
You look weak
Because look is a transitive verb, it needs a direct object. So can Weak be a direct object here? Or could it be look is a noun here?
Look in this sentence is used as a Linking verb. they link the subject of the sentence to a word or phrase in the predicate that renames or describes the subject.
If the verb is a form of be (be, being, been, am, is, are, was, were), you have a linking verb.
For other verbs, if you can replace the verb with a form of "be" and the sentence makes sense, you have a linking verb.
In "You look good", if you replace look with "are" you have "You are good", then look is used as linking verb. It is a synonym of "seem"
Examples with explanation:
John is nice. (Is links John to an adjective describing him.)
Lauren was at the theater. (Was links Lauren to a phrase telling us where she is.)
Tanner feels sick. (Feels links Tanner to an adjective describing him.)
Mr. Peters is our teacher. (Is links Mr. Peters to a noun renaming him.)
The flower looks beautiful.