"Someone else's", "Someone's else", and "Someone else's one"

Question

For example:

-- You should move your car out of the way.

-- This is not my car but someone else's car.

I want to get rid of the redundancy in the sentence which is presented by the second "car". So how can it be rephrased with being grammatically correct?

I thought of:

This is not my car but someone else's.

But there must be a possessive noun as the apostrophe + 's' don't stand alone.

Another phrasing:

This is not my car but someone's else.

I know this can't indicate the same meaning since "else" would act as the possessive noun, but would it be valid in this case?

And another one:

This is not my car but someone else's one.

Substituting "car" with "one" seems suitable; however, the sentence wouldn't sound very natural and/or totally non-redundant.

Answer 1

Your first attempt is perfectly natural and understandable.

Someone else is a noun phrase, so it's quite appropriate to simply add an apostrophe and and s after it to indicate possession.

You also don't need to repeat the noun car because it's understood by parallelism as having been mentioned at the start of the sentence.

Don't touch that car. It's Mary's car.
Don't touch that car. It's someone else's car.

In fact, while leaving the second instance of car in the sentence would be fine, it wouldn't sound as natural.

Therefore:

✔ This is not my car but someone else's.

Or simply:

✔ This is someone else's car.

Either version works, and it depends only on if you want to mention your own ownership directly or not.

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It's unidiomatic to say this is someone's else. It would be interpreted as meaning an else that belongs to someone—which is nonsensical.

And while someone else's one is not outright incorrect, is very awkward and would not normally be used.

Answer 2

What you should say, and feel comfortable saying, is

This is not my car but someone else's.

The object in question is a car, and both possessives refer to it. Suppose we replaced "someone else's" with a possessive pronoun:

This is not my car but his.

I think this makes the relationship a little clearer. The car is not mine, it's his. You really don't even need more than a dummy subject to get the idea across:

It's not mine, it's his.

Similarly, you can name the car owner, using the possessive form:

It's not mine, it's Carl's.