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.
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'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...