含有情態(tài)動詞的主動句變成被動句時(shí),由“情態(tài)動詞＋be＋過去分詞”構(gòu)成,原來帶to的情態(tài)動詞變成被動語態(tài)后,to 仍要保留.例如：
We can repair this watch in two days.
→This watch can be repaired in two days.
They should do it at once.
→It should be done at once.