33.18 Substitution zu + infinitive <=> subordinate phrase (dass)

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.

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

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