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:

  1. Metal is not easy to be digested. (incorrect) Correct - Metal is not easy to digest.

  2. Our boss is difficult to be pleased. (incorrect) Correct - Our boss is difficult to please.

  3. 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.

1 Answer 1


I am approaching these examples as a native English speaker and a graduate student studying English. I will address each of your examples separately, and then summarize.

  1. The first example is incorrect, as you have it labeled. However, in your correction, you eliminated the use of the passive voice. If you want to keep the passive voice, you need to change your verb to an adverb. For example, "Metal is not easily digested".

  2. The second example is also incorrect. In your correction, you did the same thing that you did in the previous correction and changed from passive voice to active voice. If you want to keep the passive voice, you need to change difficult to an adverb. Technically, "difficultly" is the grammatically correct adverb of "difficult", but it sounds strange and it is hardly used in spoken English. Instead of doing this, you can precede the verb with "too", because the phrase "too difficult" functions as an adverb. Thus, your example could be "Our boss is too difficult to be pleased".

  3. In the last example, the rule that we used in number two makes this example correct, so it doesn't need changed. The phrase "too weak" functions as an adverb, making the sentence "He is too weak to be moved" grammatically correct.

In conclusion, the rule that you mentioned above should actually be "you cannot use passive voice if it is to be preceded by an adjective". You can, however, use passive voice in some cases if it is preceded by an adverb.

  • Thanks for your clarification. It was really helpful, but I beg further more assistance in some of my confusions I still have. Sep 5, 2016 at 7:05
  • You may have wondered why I have changed every passive mode (labeled as incorrect) into active voice. The reason is that I'm actually asked one of this kind of sentences - take for example, This book is hard to read. - to transform into passive voice, and I got into trouble with the answer: This book is heard to be read., as the questioner showed me the rule mentioned above (as you corrected). I had given the questioner another option i.e. It is hard for the book to be read. Sep 5, 2016 at 7:06
  • Can this be correct instead? If not, I cannot understand what's the issue with if I use adjective before passive infinitive, or if I use passive infinitive after an adjective? Does it make no sense in this way? please explain. Sep 5, 2016 at 7:06
  • +1 Good answer, except that in the case of 2, I think your suggestion changes the meaning. I believe Our boss is too difficult to be pleased strays from the original meaning. I think I might have suggested Our boss is only with difficulty pleased. But I agree it is best to avoid difficultly - not a word to attempt, especially after a few drinks.
    – WS2
    Sep 5, 2016 at 8:20
  • "It is hard for the book to be read" works because hard is in a separate clause ("it is hard"). "Hard" is attributed to "it", not "book". The book is hard to be read, however, is incorrect. Sep 5, 2016 at 10:52

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