You do not need to be proficient at the grammar of a language to read and write it fluently. For example, the difference between a sentence, phrase and clause is almost inconsequential; at the essence, those three terms all mean “a group of words”, despite having technical differences when discussing grammar. Most people can get by with just the basic knowledge of sentence construction, or even with just the combination of a few words and body language.
Basic sentences and body language does not make for efficient or thorough communication. Many details might not be expressed and thus affect the effectiveness of communication. With grammatical knowledge, more complex sentences can be formed and in spite of little errors, the general meaning can be conveyed more adequately. There’s a difference between bad grammar in a text message or an online forum as compared to a formal publication like a white paper or news article. I can accept your spelling and grammar issues when you punch out a quick response with your thumbs. When I’m trying to learn something, I expect the author’s position to be clear and easy to parse.
[P1] People can communicate with basic sentences and body language.