Somewhere I read this kind of rules - "you cannot use passive infinitive if it is to be followed by an adjective." Take some following examples:
Metal is not easy to be digested. (incorrect) Correct - Metal is not easy to digest.
Our boss is difficult to be pleased. (incorrect) Correct - Our boss is difficult to please.
He is too weak to be moved. (incorrect) correct - He is too weak to move.
--- ????? Are all these true? However, I have doubt. Please clarify it. Thanks! in advance.