When I started out I used to teach gramma points from books and most of the time it seemed new to the students. Yet, there still seemed those who knew whatever it was. These people were generally unable to use the grammar and so the lesson was more of a revision/activation/clarifying lesson.
Now that I look back I think that far more of those students knew the grammar but were too polite to say. Think about it!! If you have a 20-year-old student who’s been learning English for 10 years they will have covered most grammar.
As a result, your input may not be as new and/or useful as you think.
This is one reason why I’ve heard directors/co-ordinators say that “students have learned all the grammar already, we are ready to activate it and build on it”. This sounds great but aren’t most materials made for ‘teaching’ and even our methods too??