Why is the sentence car incorrect?

The word "car" isn't inherently incorrect. It's a perfectly good noun meaning a four-wheeled motor vehicle. It's only incorrect if it's used *incorrectly* in a sentence. For example:

* Incorrect: Car go fast.

* Correct: The car goes fast.

The first example is incorrect because it lacks proper grammar (subject-verb agreement and article). The word "car" itself is fine, but its *usage* is flawed. To say why "car" is incorrect, you'd need to provide the *sentence* containing it.