[A] assume, believe, consider, feel, know, suppose, understand pueden ir seguidos de objeto + to be:
- I consider him to be the best candidate.
Pero es mucho más corriente usar that + un tiempo ordinario:
- I consider that he is the best candidate.
Con think, estimate, y presume la construcción de objeto + infinitivo es extremadamente rara, usándose normalmente una that-clause en su lugar:
- I think that he is the best player.
- They estimate that this vase is 2,000 years old.
[B] Pero cuando estos verbos son usados en la voz pasiva van seguidos con más frecuencia por un infinitivo que por una construcción con that:
- He is known to be honest = It is known that he is honest.
- He is thought to be the best player = It is thought that he is …
- This vase is estimated to be 2,000 years old.
[C] Notar que suppose, cuando se usa en la pasiva, transmite a menudo una idea de deber:
– You are supposed to know the laws of your own contry =
= It is your duty to know/You are expected to know …
[D] Puede también usarse el infinitivo continuo:
- He is thought to be hiding in the woods. (People think he is hiding.)
- He is supposed to be washing the car. (He should be washing it.)
[E] Cuando el pensamiento concierne a una acción previa usamos el infinitivo de perfecto:
- They are believed to have landed in America. (It is believed that they landed.)
suppose + infinitivo de perfecto puede transmitir o no idea de deber. «They are supposed to have discovered America» significa «Se piensa que lo hicieron.» Pero «You are supposed to have read the instructions» normalmente significaría «Deberías haberlas leído.»
(Para construcciones de infinitivo después de verbos pasivos, ver también luego.)
A Practical English Grammar (A.J. Thomson & A.V. Martinet) 