If the subordinate phrase has the same subject as the main phrase the subordinate phrase can be changed into a infinitive construction and vice versa.

Examples     Er glaubt, dass er es tun muss. <=> Er glaubt, es tun zu müssen.   He believes that he has to do it.   Er meint, dass er es weiß. <=> Er meint, es zu wissen.   He thinks that he knows.

A transformation is not possible, if the subordinate clause has an own subject.
Er glaubt, dass wir es wissen. <=> ø