Which of the following is not an example of a rule of syntax?

To pronounce the plural of a regular noun, say the noun and add $"S"$ if the noun ends in a hard consonant and add $"Z"$ if the noun ends in a soft consonant or vowel

All sentences must have a noun phrase and a verb phrase

Singular nouns $($e$.$g$.,$ "book"$)$ are preceded by an article $($e$.$g$.,$ "the"$)$

An adjective $($e$.$g$.,$ "difficult"$)$ precedes the noun it modifies $($e$.$g$.,$ "test"$)$