Why doesn’t YES teach English grammar?
Understanding the physics of mass, force, trajectory – the scientific behaviour of objects in motion – is not much help in learning how to play a ball game like football or netball. You need to practise playing the game. If you do that you will learn a great deal about motion physics, but it will be an understanding that is real and practical, not theoretical.
In the same way, you can’t help but learn a lot about a language’s grammatical structure when you practise using that language. That’s how we all learned the rules of our first language. You might not realise that is what you are learning, but if you know which English words to use and how to use them in different situations, then you have an intuitive and much more useful knowledge of English grammar than you can get from a textbook.
Studying a language, its structure, its syntax, its rules, its origins – what academics would call linguistics – can be fascinating, but it has very little to do with learning how to speak a language.
The only way to learn how to speak a language is to speak it – and to get lots of practise.