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The car is the same as the other one, but just with a spoiler.

What does 'but' join? The prepositional phrase 'with a spoiler' to 'as the other one'?

Is this below more idiomatic, preferred?

The car is the same as the other one, just with a spoiler.

Comments  
Yes, the second version is better. "but just" / "except only" is bad form - perhaps redundant - definitely juvenile.

Afraid I'm not up to your first question.

It's clearly different in structure from "It has everything but / except the spoiler," where "but" is a preposition or an adverb.
I guess you're calling it a conjunction in your example. (Probably another reduced phrase/clause.)Emotion: smile
the same --- but (here's how it's not the same) with a spoiler

So but joins same and with a spoiler.

Compare:

Compared to the story we read yesterday, this story is the same, but without the introduction.

CJ
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Avangi, thanks for your thoughts on 'but just' being poor writing. I wonder why this is so. I do agree it sounds cumbersome, but one is a conjunction; the other an adverb, so I can't see why it would be redundant.

CJ, the reason I found this one difficult is because I have the understanding that conjunctions must join equal sentence parts. This one does not. Do I have it wrong, or is this an exception to the rule, or what?
English 1b3I have the understanding that conjunctions must join equal sentence parts.
It's time to change your mind! Much is omitted in statements of the kind you presented earlier. The word same is the problem. same stands for the negation of the prepositional phrase with a spoiler.

That car does not have a spoiler. This car is the same, (that is, also a car that is not with a spoiler), but (this car is) with a spoiler.

Not grammatical, but illustrative.

CJ
CalifJimMuch is omitted in statements of the kind you presented earlier.

Good point. I think I just forgot this fact when asking the question.
CalifJim
That car does not have a spoiler. This car is the same, (that is, also a car that is not with a spoiler), but (this car is) with a spoiler.

This shows the missing words. It also shows that but is in fact joining two main clauses. Thanks (I'm thinking aloud, so you can correct me if...)

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English 1b3This shows the missing words. It also shows that but is in fact joining two main clauses.
That's right.

Emotion: smile
CJ