I have searched lots of websites to understand which one is correct in this sentence:

It is too hard to (be) explain(ed).

Some people say that after some adjectives called tough adjectives you can't use the passive form of verbs. I have heard from some native speakers that It is too hard to be explained is very strange to their ears. So I search on the Internet and I found that at least 14 million people think that to be explained is correct and they actually have used it. I am sure that I am missing something here. Maybe they are used in different situations. But I couldn't find any real difference between them.

Any help is more than welcomed.

P.S: To further complicate the situation for a word like pass, when I searched it on the Internet, you can't really say "Exam was too difficult to be passed".

  • 1
    The phrase you're looking for is "Tough-Movement predicates", like tough, easy, hard; Tough-Movement is a rule that moves only direct objects from complement clauses. Notice that Calculus is hard to explain is grammatical (Tough-Movement), while "Calculus is hard to be explained" is not. This isn't about hard or about Tough-Movement; it's about the too X to VP construction, which means 'so X that Not S', and can select either subjects (He's too short to reach the ceiling) or objects (He's too short to pick for the team). Commented Feb 21, 2015 at 18:03

1 Answer 1


These Google Ngrams show that ... too tough to be eaten ... and ... too tough to eat ... are both used, but that the second variant, with the tough movement, is far the more common.

I'd say that ... too hard to be explained ... is considered stylistically even less optimal. The relevant Google Ngrams seem to confirm my suspicions.

Too difficult to be explained has a very small following.

This Wikipedia article looks at tough movemment.

  • So, the reason that I can't find a sentence like "exam was too hard to be passed" is that usually, kids use a sentence like this and they are using more modern forms of language.
    – Masoud
    Commented Feb 21, 2015 at 10:18
  • Some phrases like 'to be eaten' or 'to be passed' or 'to be explained' aren't used in books anymore. But they are being used in everyday speaking.
    – Masoud
    Commented Feb 21, 2015 at 10:29

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