这本书是英语语法和用法的宝贵指南。

This book is a valuable guide to English grammar and ___usage.

这就是为什么我无法解决所有英语用法上的问题。

That is why I couldn't solve all my problems with English ___usage.

掌握习语的用法是听起来像母语者的关键。

Mastering the ___usage of idioms is key to sounding like a native speaker.