I know that it must be my father.过去式应该是...
3个回答

一楼的翻译是把it当he了.

这两句都是对的,关键看你在具体语境下具体想表达的意思了.

I knew that it must have been my father.

我就知道这事儿一直都是我父亲在做.

I knew that it must be my father.

我就知道那是我父亲.

区别就是一个是说某事的延续,一个是说某事的时点.