WHY is this sentence incorrect?

"All that they can do is preparing as much as they can."

I know it should be

"All they can do is (to) prepare as much as they can."

But, for the life of me I can't figure out why.

  • Would you write "They can preparing°? – Michael Harvey Oct 25 '18 at 16:45
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    The reason is that in reversible specifying constructions like this, if the subject noun phrase contains "do" or "did" in a relative clause, the complement is restricted to an infinitival, either bare or with to. Compare "All they were doing was preparing as much as they could" – BillJ Oct 25 '18 at 17:13
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    Note, however, that if it is reversed, the gerund is OK and the infinitive is awkward. I think this has to do with gerunds being more comfortable as subjects than infinitives are. – John Lawler Oct 25 '18 at 17:42
  • @JohnLawler But "To err is human...", I feel sure you would agree. – WS2 Oct 26 '18 at 0:28
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    Of course. Complementizer choice is a morass of details and exceptions. – John Lawler Oct 26 '18 at 2:02

'Prepare' is grammatically parallel to 'do' and should have the same form. (They can prepare as much as they can.)
If for instance you wrote : What they like doing is... then it would be the -ing form; what they like doing is preparing...

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