[weiming]

I love language and linguistics in general but books on grammar put me right to sleep. If a systematic grammatical error comes up, fix it. If a rule can be identified, do it and learn it, otherwise forget grammar.

Ideally we would just pick up grammar naturally as we learn, unfortunately when we are learning a second language we apply our grammar to the new language. Here it needs to be corrected.

For native speakers, most don't really know or understand grammar, they only know "what sounds right" or "what people say, and what people don't say". For a learner it depends on how much exposure they have to the native language, and why they are learning the language (to talk with people informally, or to take exams and communicate formally).