“You can say, ‘a circle is rectangular in shape,’ and all you’ve done is confused us!”
Serious question:
- Should it be all you’ve done is confused us?
- Or should it be all you’ve done is confuse us?
I don’t know what rule governs this situation. But my upbringing (thank you to my mother, Leona Anne, of Ashland, Ohio) suggests a rule something like this:
After a phrase like all you’ve done is — you switch to the present tense: confuse us!
But why? I don’t know. Maybe the phrase all you’ve done is actually stands in, as a substitute, for an implied longer phrase: all you’ve done is, you’ve… In which case, you need a past-tense verb: you’ve confused us!
Somebody help me out here. Use the comment box and enlighten all of us, please.
English Is A Complicated Language 
I think maybe this is one of those situations where the infinitive is what is called for, but with the “to” suppressed (sometimes called a “bare infinitive”) … so “all you’ve done is (to) confuse us.” So – to get all grammary on ya: Infinitives function as nouns – so the phrase “to confuse us” is (I think) functioning as the predicative nominative of “all” … A (All) = B (to confuse us) … The word “is” functions like an equal sign … so since “All” is a noun, the thing on the other side of the equation must also be a noun … rather than the verb-y sort of structure of “confused us” … right? The sentence could also be written “All you’ve done is to have confused us” – again, getting the infinitive in there straightens things out. So I agree with your mom.
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This smells right to me. Thank you, Rebecca!
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