What grammar rule is referenced by this saying? "Do not work hard; work effectively. Not only is it grammatically correct, but it will also help you be more successful."
The grammar rule here is that you must use an adverb to describe a verb.
From a grammatical standpoint, the phrase “work hard” is not correct because “hard” is considered an adjective, not an adverb. On the other hand, “effectively” is an adverb. You use “effectively” to describe the verb “work.” Of course, this is also good advice because you need to work effectively, rather than working hard.
Another grammatical rule followed in this sentence is the semicolon. You use the semicolon for a compound sentence between two independent clauses. The use of semicolon is also correct in this sentence.