Grammatical rules are the guidelines that tell us how words and sentences should be structured in a language to make them understandable and correct. They help us form proper sentences and avoid mistakes in our writing and speaking. Full definition
Similarly, infractions of grammatical rules and improper punctuation, including capitalization and run - on sentences, can mean the difference between job interviews and a phone that does not ring. (njresumeservice.com)
For example, some broken grammatical rules are acceptable, such as the use of sentence fragments to make the writing crisp. (vault.com)
Some birds seem to have grammatical rules in their songs. (discovermagazine.com)