有情态动词时主动语态怎样变为被动语态,例句

含有情态动词的主动句变成被动句时,由“情态动词＋be＋过去分词”构成,原来带to的情态动词变成被动语态后,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.