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Why do we say “supposed to” for “should have”?

I guess I really can't tell if 'should' and 'be supposed to' are interchangeable from the definitions of the dictionaries. Can anyone prove they are synonymous or not, with evidence?

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marked as duplicate by Uticensis, JSBձոգչ, kiamlaluno, MrHen, RegDwigнt Apr 28 '11 at 18:47

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Should in the modern sense means "ought" or that you would do something, but are unable to. As in, "I should clean my room, but I'm tired." The archaic sense, as in owing an obligation, has mostly been replaced with the singular simple present: shall.

The phrase "supposed to," when paired with an infinitive suggests an obligation or an intended purpose. Such as, "I'm supposed to clean my room," or "A clean room is supposed to look nice."

So, in modern usage, "supposed to" suggests owing an obligation as opposed to "should" which, in itself, has become a suggestion.

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"should" or "ought to" in linguistic roots stems from the Indo-European optative mood, indicating a wish or hope - "we should make it home by morning", whereas "supposed to" is in the indicative mood, stating fact - "I am supposed to be at work in the morning".

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While there is nothing actually false in this answer, I believe it is about as unhelpful as you can get without falsities. Both "should" and "supposed to" have both epistemic and deontic meanings, but you have chosen one for one, and one for the other. And the fact that a meaning of "should" sometimes corresponds to a meaning of the optative in those languages which have such a form is completely irrelevant to any account of the form in English. –  Colin Fine Apr 28 '11 at 17:08

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