情态动词全解析：态度前缀、易混对子与四大功能分类

情态动词是加在动词前的"态度前缀"：can与could、will与would、must与have to到底怎么区分？must have done、should have done、needn't have done又如何对过去"事后评价"？一文梳理情态动词的四大功能分类与核心易混点。