主系表：be 動詞是系動詞.例句：
I am a student.He is at home.We are busy.She was in the classroom a moment ago.
進行時：be 動詞為助動詞,后面接動詞的現(xiàn)在分詞.例句：
I am studying English.He is doing his homework.They are playing basketball.
被動語態(tài)：be 動詞為助動詞,后面接及物動詞的過去分詞.例句：
English is taught in our school.Trees are planted in spring.The room was cleaned yesterday afternoon.
希望解釋明白了.