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I think that it's not so much a rule as a bit of common sense. If it's a statement then said would be the word. if it's a question then the person asks about it and if they are uncertain about something they may ask about it. Using the wrong one can be really jarring. Like using its when a person is supposed to use it's i.e 'Its getting late,' instead of 'it's (it is) getting late.'
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