Hello **** It seems to be a witticism, with a play on the word 'rules' ('rules' = noun, and 'rules' = 3rd person singular of the verb 'to rule'). At first, the sentence sounds nonsensical, as we expect 'them' (for 'rules' = noun) instead of 'it'. Then we realize that 'grammar rules' = 'subject + verb', and that 'remember grammar' means 'remember that grammar'.
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