The ’s can be used as a contraction representing a weak, unstressed word that is not pronounced. It allegedly cannot occur in sentence final position.

  • She is not ready, but he is.
  • She’s not ready, but he is.
  • She’s not ready, but *he’s.

The last one is not grammatical there.

Similarly, here the last one is not grammatical:

  • He has not started yet, but she has.
  • He’s not started yet, but she has.
  • He’s not started yet, but *she’s.

This answer claims this is because you cannot end a sentence with a weak form. It backs up its point with reasonably scholarly documentation.

However, this does not seem to be invariably true.

  • Shall we go to the movies? Yes, let us go.
  • Shall we go to the movies? Yes, let’s go.
  • Shall we go to the movies? Yes, let’s.

Unlike the third example in the first two sets, here in this case the third example is indeed grammatical, despite ending with a weak form.

Is this an exception to the rule, or is there another rule at work here?

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    Doesn’t the rule say that the weak forms of is and has may not occur in final position, rather than that ’s may not do so? – Barrie England Dec 19 '13 at 17:53
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    I seem to remember that let's is an example of grammaticalization, but I don't think that applies to he's, she's etc. Apart from anything else, the contracted part in let's is the all-important pronoun us (which has true meaning in context), whereas in the others it's just an almost superfluous is (simply a "helper verb" required by grammatical considerations). – FumbleFingers Dec 19 '13 at 18:28
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    I think the correct formulation of the rule is: you can't contract is, are, has, had, or have at the end of of a sentence. – Peter Shor Dec 19 '13 at 18:41
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    @Peter,tchrist: It seems to me rule is more like you can't contract auxiliary verbs at the end of a sentence. But if indeed the permissible forms (as opposed to the unacceptable ones) can be fully defined by "rules", it's not obvious to me why we only accept My cellmate said, "Let's have a party tonight". But the prison guards wouldn't let us. Where expanding the first instance would be hopelessly archaic, and contracting the second would be a complete no-no. – FumbleFingers Dec 19 '13 at 20:51
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    I think your interesting example shows that "let's" is not a contraction. – Greg Lee Mar 24 '16 at 16:16

It's worth noting that they're contracting different things:
He's/She's = He is/She is (not he has/she has, as you use it in the second set of examples)
Let's = let us.

So "he's" is a contraction of the form N+V while "let's" is V+N.

Other N+V contractions can't end sentences:"
"Is there a dog here?" "*Yes, there's"
"Is that a mammoth?" "*Yes, it's" "Will you take me to the store?" "Yes, I'll" "Are you santa?" "Yes, I'm"

But contractions with the verb first, you can:
"Should I jump over this alligator?" "No, don't!"
"Will you go to the moon with me?" "No, I won't"

And possessive 's can also end sentences:
"Why is that cat staring at me?" "Don't worry man, it's Tom's"

So it seems like the only contractions that can't come at the end of a sentence are those which the contracted portion is a verb!

Hope that helps!

  • I think you’re onto something, but I think it only involves auxiliary verbs, whether modal or otherwise. – tchrist Dec 19 '13 at 22:15
  • Totally. I can't think of a contraction that uses a non auxilliary verb actually. – McGurk Dec 20 '13 at 2:35

The idiomatic expressions "let us . . ." / "let's . . ."

meaning roughly

"here's an idea; I strongly suggest that we . . ."

are, I think, always interchangeable on grammatical grounds (though some older Church leaders might consider "Let's pray" over-informal, and often sentences like "Let us go to the cinema tonight" would sound far too stilted).

However, "let us . . ." meaning "allow us to" is never rendered "let's". Idioms often seem to have their own rules.

Very late answer, I'm sorry. (I was doing some googling about this and couldn't find anyone who'd written out the whole rule, so let's try this.)

Contractions of the form xyz + to be (in that order) only appear with a predicative expression following.

You can look up what a predicative expression is, but here's some examples:

  • He's {ready}.
  • She's {a firefighter}.
  • They're {at home}.
  • That's {great}.

You can think of the predicative expression as the supplement to the verb to be.

The uncontracted form is always possible, the contracted one isn't. When there's no predicative expression following, you don't contract:

  • That's {how it is {}}.
  • Now's {the time to party}, right? -- Wait a second.. Now is {}.

That's not restricted to the end of a sentence:

  • I don't know where he is {} all the time.

all the time is not the predicative expression here, that role goes to where. In contrast, we have:

  • He's {where exactly}?
  • I don't know where he's {going} all the time.
  • I don't know why he's {grumpy}.
  • I don't know why he's {leaving}.
  • That's {all the time we've got in our show today}.

Analogous rules work for contractions with will, would, have, had.

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