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What is the actual spelling/pronunciation? What is the origination of this phrase?

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Supposed to is a very common phrase, as in: you're not supposed to come here. It means "you should not come here", "you are not meant to come here". Using *suppose to in this sentence would be considered wrong by most educated speakers. The cause of this common omission of the d is probably that d t sounds the same as t, so that there is no difference in pronunciation between supposed to and *suppose to.

To suppose something means to assume something. When used in a passive construction, you use a form of the verb to be plus the past participle of to suppose, which is supposed. To be supposed to means to be expected to, to be obliged to. You can see that the verb gets a somewhat different sense in the passive voice from what would normally be expected.

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    Good explanation. +1 – Robusto Jan 7 '11 at 2:23
  • It sposta be bad juju to spell it without all the silent letters because you'll have bad luck for seven years. – John Lawler Feb 11 '17 at 0:16
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    It's always sposta in speech. Orthography is irrelevant. – John Lawler Feb 11 '17 at 3:18
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    @JohnLawler, Cerberus: I don't think it's an "absolute" (not much is, when it comes to language use! :) but I have the feeling contracted sposta is far more tightly wedded to deontic contexts. Trouble is I get confused when I apply that thinking to John's first comment, which appears to contain elements of both senses (as with You're sposta drink this 'cos it's sposta be good for you). All I know for sure here is that to my ear, John's comment would sound a bit odd with fully-articulated supposed to, but that would be perfectly okay with your (Cerberus) last comment. – FumbleFingers Mar 15 at 16:17
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    @JohnLawler: To my "inner grammarian", Language is supposed to be innate strongly implies People think the ability to use language is something humans are born with, whereas Language is sposta be innate smacks more of People should use language in whatever way seems natural to them. Maybe innate isn't the best word to exemplify "straddling both senses", but you catch my drift. – FumbleFingers Mar 15 at 19:16
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The original meaning of "suppose" is epistemic: "think", or "assume".

But various modal verbs, such as "must", have both deontic ("you must go" = "you ought to go") and epistemic ("you must be" = "I conclude that you are") meanings.

In the passive only, "suppose" has acquired a deontic sense.

Nowadays "suppose" nearly always takes a sentential complement ("I suppose that ... "), but in older or more formal English it can take a direct object and an epistemic infinitive complement "I supposed him to be ... ".

So "He was supposed to go" is the passive of a construction which is now rather restricted, in the epistemic sense of "It was thought that he was going". But in the specialised deontic sense, the construction has survived.

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The expression "supposed to", when used to describe what someone should do, is always spelled "supposed to" and always goes with a form of the verb "to be" (E.g. "they are supposed to do X", "I am not supposed to do Y").

Historically, it originated as a passive-voice version of the verb suppose, meaning in this context "expect". The "supposed" part is therefore in origin a past participle, which is why it has the "-ed" suffix.

But in modern English, this expression doesn't really behave like an ordinary passive-voice version of to suppose. For one thing, I believe everyone pronounces it differently: "suppose" is /səˈpoʊz/, ending in voiced /z/, while "supposed to" is /səˈpoʊstə/ or səˈpoʊstu/, with voiceless /s/. This kind of phonemic voicing assimilation between words is unpredictable and not common in English; so I think "to be supposed to" should be classified as fixed expression that has become somewhat distinct from the ordinary verb "suppose", just as "to be born" has become somewhat separate from the ordinary verb "to bear".

We see the same devoicing of originally voiced fricatives in certain other verbs that have developed specialized meanings in fixed expressions containing the word "to":

  • "have to"/"has to": normally, "have" ends in /v/ and "has" ends in /z/, but "have to" has /f/ and "has to" has /s/. (But "had to" retains a voiced /d/ phoneme, which is not a fricative.) This only applies to "have to" as a fixed expression with a specific, special meaning; it doesn't occur when the word "have" or "has" just happens to come before "to", as in "She will give all she has to fight cancer."
  • "used to" meaning "once did" has /s/, even though the ordinary verb "use" has /z/.

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