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This is the kind of mistake which the teachers haven't been able to prevent the students from making.

Which the teachers haven't been able to prevent the students from making.

S= the teachers , V= haven't been,

able is an adjective , so I think it should be a complement in the sentence.

So far, we have S V C, which is a normal grammar sentence structure.

But what about the infinitive " to prevent the students from making"

Is this another complement? But S V C C structure doesn't seem to exist.

Or is the infinitive an adverb modfiying C(able), so it remains S V C(+adverb clause)?


By the way, the PP "from making " is an adverb modifying the verb "prevent" and object of the PP is "mistake", is that correct?

  • to be able to is a set phrase. You might want to rethink your parse. – Lambie Feb 7 at 17:07
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This is the kind of mistake which the teachers haven't been able to prevent the students from making.

Strictly speaking "been able to prevent ... " is a separate clause serving as complement of “haven’t". "Been" has the adjective phrase "able to prevent the students from making" as its complement, in which the infinitival clause "to prevent the students from making" is complement of "able".

"From making ____ is complement of "prevent", where the gap notation '___' represents "which", the direct object of the relative clause, which has the nominal "kind of mistake" as its antecedent.

Arguably, the noun phrase "the students" is a 'raised' object, i.e. the verb it relates to syntactically is higher in the constituent structure than the one it relates to semantically.

  • I doubt most people here can understand your post. It would help to explain it and put things like raised object in parenthesis. – Lambie Feb 7 at 17:06
  • Thank you so much for the answer. But I don't understand " the verb it relates to syntactically is higher in the constituent structure than the one it relates to semantically." – Peilin Feb 8 at 15:13
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    @Peilin "The students" is the syntactic object of the verb "prevent". And it is also the understood subject of the subordinate clause "making (the kind of mistake)". The subordinate "making" clause is subordinate to the "prevent" clause, i.e. it is lower down in the syntactic structure, so although we think of "the students" as being the subject of "making", it is 'raised' to object of "prevent", which is higher in the structure. I said "arguably" in my answer because some speakers maintain that "the students" is an argument of "prevent", and thus is an ordinary object, not a raised one. – BillJ Feb 8 at 15:43
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Able is an adjective, but in the construct be able to it is a verb. Or acts like a verb, depending on who you talk to[1]. You use it in situations where you can't use can due to that verb being defective. A verb is defective when it cannot be fully conjugated, that there are some moods, tenses and forms that it does not posses.

You might say "the teachers can prevent the students..." or "the teachers can't prevent the students...", but you can't shift can into all tenses or forms. In those situations, you use to be able to. Note that those forms missing from can include the infinitive itself.

So the principle verb here lies in the phrase "haven't been able to, and the object is "prevent the students from making".


[1] People will argue about the technical categorisation of this, but for the purposes of just learning to use it (and according to some expert sources), to be able or to be able to is a verb. It conjugates via the to be part.

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