Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
We've no time to lose.
They'd rather not do it.
They'd finished the job.
Let's go.
Who's on first?
Who's got the time?
'Tis a pity.
It's a pity.
It's already been done.
[we have]
[they would]
[they had]
[let us]
[who is]
[who has]
[it is]
[it is]
[it has]
These types of contractions are not appropriate for all genres of
writing: In particular, they are often viewed as too informal for
academic journals (which isn't to say they may never be used there,
just that they must be used with discretion). On the other hand, they
are fine for informal writing and virtually mandatory for dialogue, as
speech would sound ridiculously stilted and unnatural without them.
In fact, in dialogue you can (provided you don't overdo it) even get
away with double contractions such as "You shouldn't've said that"
or "I'd've known him anywhere."
Different words may sometimes form the same contraction. This
doesn't usually create a problem, since the context should make
your intentions clear, but occasionally you might need to spell things
out to avoid ambiguity. For example, your readers might not be able
to distinguish whether I'd let means "I would let" or "I had let."
SINGLE-WORD CONTRACTIONS
Single words are sometimes shortened to make them less formal.
Letters may be dropped anywhere in a word: from the beginning,
the end, the middle or both ends.
For some such contractions, an apostrophe is mandatory. Since
the contraction is not a "real" word, the apostrophe is needed as a
signal that something is intentionally missing. If you omitted it, it
might look as though you were simply ignorant of how the word
should properly appear.
192
So, how's life in the 'burbs?
How are you doin'?
Please hurry, 'cause we're late.
C'mon, let's go.