How is the sentence dog ran down street ungrammatical?

The sentence "Dog ran down street" is ungrammatical because it lacks a definite article before "street". It should be:

* The dog ran down the street.

Here's why:

* Definite articles (the, a, an) are necessary to specify which noun you are referring to.

* In this case, "the street" implies a specific street that the dog ran down. Without the article, it's unclear which street is being discussed.

Other ways to make the sentence grammatically correct:

* Dog ran down a street. (Implies any street, not a specific one)

* A dog ran down the street. (Implies any dog)

By adding the correct article, the sentence becomes clear and grammatically sound.