A friend of mine who is a linguist told me one time that most native English speakers have never had a course in English grammar. They’ve had courses in the pet peeves of English teachers, but have never seen a systematic description of the structure of the English language.
See also xylophones and zebras, learning Spanish.
Most folks who only know their native tounge have never had a course in grammar.
A friend who teaches ESL (English as a Second Language) says this is the biggest stumbling block for those who for example only speak Spanish. If you’ve never studied a grammar, especially if you are not very comfortable with parts of speech, it is difficult to learn a new language when it is presented in those terms.
I recall my Dad talking about “International English” spoken at international scientific conferences — attendees who were primary speakers of different languages usually communicated in a much simplified and stripped down version of English. One of the big simplifications was reducing the vocabulary to something like 1000 words plus jargon, but grammar was much simpler, too.
The other day I noticed there is a Wikipedia in “Simple English” just like there are Wikipedias in various languages. It looks like the idea is to write articles in something like International English, but the target audience includes children along with non-native speakers.