主系表：be 动词是系动词.例句：
I am a student.He is at home.We are busy.She was in the classroom a moment ago.
进行时：be 动词为助动词,后面接动词的现在分词.例句：
I am studying English.He is doing his homework.They are playing basketball.
被动语态：be 动词为助动词,后面接及物动词的过去分词.例句：
English is taught in our school.Trees are planted in spring.The room was cleaned yesterday afternoon.
希望解释明白了.