When is it ok to create a contraction of words followed by “s”?

When is it correct to create a contraction of words followed by is? For instance is who’s a correct short form of who is?

Solution 1:

The word "is" can always be contracted, provided it is not stressed - though this is considered somewhat informal.

So, in informal contexts,

Fred is taller than Jim.

can be shortened to

Fred's taller than Jim.

...it doesn't depend on what word comes before "is". But if the is is emphatic:

Fred is taller than Jim.

then it would be wrong to contract that to

* Fred's taller than Jim.

That is probably fairly obvious - if you are stressing a particular word, it doesn't pay to shorten it. But there are also other situations where the is has the emphasis, for instance

I don't know what it is.

cannot be contracted to

* I don't know what it's.

even though

I don't know what it is doing.

can be reduced to

I don't know what it's doing.