This rule is important for the correct understanding of grammar.

I bought a new grammar book.

He spoke grammatically correctly.

This rule is of grammatical importance.

Grammatical rules are important for learning a language.

The grammaticality of a sentence depends on the correctness of its grammatical structures.

That person's exaggerated grammatical pedantry is really annoying.

Grammar rules are important for learning a foreign language.

The bad grammarian made many mistakes in his last composition.

He spoke grammatically correct.