Can “it is” always be shortened to “it’s”? [duplicate]

Solution 1:

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!

Solution 2:

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.