What Kevin told you about himself wasn't true
if what is a subject, what about Kevin??
I want to know there is a grammar for this sentence?? if there is what do we call it??
"What Kevin told you about himself" is a noun phrase. That combination of words is the subject.
a word or group of words that functions in a sentence as subject, object, or prepositional object.
It doesn't look like a noun, but its meaning describes a subject. You could rephrase it as "The thing Kevin told you about himself."
What Kevin told you about himself wasn't true.
This is called a 'fused' relative construction, where the antecedent and the relative word are combined, or fused, together into the single word "what".
It can be paraphrased as "The thing that Kevin told you about himself wasn't true".
"What Kevin told you about himself" is a noun phrase, as evident from the possibility of subject-auxiliary inversion, cf. "Was what Kevin told you about himself untrue?".