Why is D. incorrect?

Let's be honest here. The only sentence that sounds good is B. All the others sound stilted and weird. No one would say any of those. "In order to pay the mortgage, he had to sell his new car." would be the best way to say it imo

All of those sentences are horrible and are terrible examples for learning. "He sold his car to pay the mortgage." Sussing out the incorrect grammar here is a pointless exercise because none of them would ever be used in speech or in writing. (B) is probably closest to anything you would hear in the wild. As a native English speaker, they make my ears bleed.

The correct form is, "He sold his car to pay the mortgage." A is terrible form. Everything is out of order, and it seems like it had to be patched together B is acceptable. Delete the word why, and you've got a pretty good sentence. C is poorly worded, but clear enough. The tenses are a little confusing on this one. D is a bit of a mess. Again, if you put the facts in the right order it's easy to see the logical connection between them. If you don't, you're doing gymnastics trying to connect everything.

This isn’t a grammar problem. D doesn’t mean the same thing as all others. D is perfectly grammatical. It’s just semantically awkward. “Mortgage was why he sold his new car.” Would be perfectly fine. I don’t think this is a well-designed problem. Native speakers would simply say “he sold his car to pay the mortgage.”

I think all of these are okay. But they are awkward. The most natural way to say this would be, "He sold his new car to pay the mortgage."

The "to" in the infinitive often does 'double duty' in marking the infinitive and marking an arrow of time/purpose. It can signify "in order to", "going on to", or "up to the point when there is the opportunity/obligation/appropriateness to". All that in one little word, bundled up in superposition until context clarifies. In the context of this exercise, a native speaker will readily understand "[in order] to pay..." and infer purpose. The simple gerund does not imply an arrow of time: the default interpretation is simultaneity. You need to add a preposition like "for" to imply purpose, unless the broader context makes things clear.

As a native speaker, I totally take for granted how much grammar just “sounds right” to me. And D just sounds awkward. The cause and effect is backwards, so this isn’t how people talk or think about language. Though A sounds awkward to me because of the order of cause and effect too. I think D is probably wrong because the tense of “paying” is wrong. B is better than A and D, but I also wouldn’t say that—it’s too wordy. C is the only one that sounds remotely natural to me

Sentence C is grammatically incorrect because the tenses of *did* and *is* do not agree. This would be acceptable in informal or spoken English, but “what he did ... was,” sounds more correct to me. A ngrams search confirms: “what he did was” is 120 times more common than “what he did is” in written English. I don’t think this is a grammatical error, but I would strike the word “why” in sentence B. It sounds better without. Sentences A and D look correct to me. I don’t understand the changes your teacher is making to them.

They're all awful

It’s the distinction between a reason and a purpose - though both are the answer to a “why?” question, there are subtle differences. Note that the other three use “to” which is the preposition we would use to indicate a purpose. The way D uses “why” indicates it is a reason. A reason is something that happens that causes or leads to another event happening. Event A happens, then event B happens because of it. A purpose is when there is some need or intention - it is doing something because we want to cause a certain effect. We do A because we want/need B to happen. There was a need TO pay the mortgage, he sold the car in order TO pay it, ie to cause paying the mortgage to happen. D suggests “paying the mortgage” was an event which was the reason that “selling the car” then happened.

Personally I’d sat b is also incorrect. I know I’m nitpicking, but it should be the reason *that* he sold his new car was to pay the mortgage. Or just “the reason he sold his new car….” It’s something a native speaker might say, but that doesn’t make it correct!

D has disagreements in tense, as well as phrasing that is awkward. You could say: To pay the mortgage, he sold his new car. Or He sold his new car to pay the mortgage.

People giving “he sold his car to pay the mortgage” as the only way a native would express it are plainly wrong. There’s literature and art, and humor and god knows what else that has a place for unusual and unconventional sentence structure. For basic learning of course the plainest the simplest constructs should be used, but come on, any language is so much more than just flavorless SVO. D is incorrect, the rest are perfectly fine ways to express it, given the right context and usage.

All the 'sentences' also lack periods.

I could be wrong, but I feel that D is incorrect just because it does an ellipsis with "the reason" when it may not be obvious enough. Otherwise I'm unsure about why it could be wrong.

C. A, B and D sound awkward, but I'm pretty sure it's past tense, so I say C is wrong.

I thought C is incorrect? It should be either "is to sell" or "is selling", not "is sell".

I would not say any of these things. I would say "He had to sell his car to make his mortgage payment." (I would also wonder why this poor idiot didn't take his housing expenses into account before signing a contract on that stupid car, but no one asked for financial advice here.)

I would not ever, _ever_ use D. You need something that goes with “why he sold his car.” For this, you need a verb. Why did he sell his car? *To pay for __.* Notice how in all the other options you have “to pay”? In option D, you have a gerund (“paying”), which is functioning as a noun. You need a verb here.

i agree with your teacher. people might say d especially in casual speech, but it doesn't sit well with me. it's got something to do with what works with 'why' and what doesn't. but i'm kind of struggling to define it.

Hi everyone, I can’t edit the post but here is more context, I thought it was maybe technically correct but sounds unnatural, but this is what my teacher says would be how to fix this sentence, “Why he sold his new car is pay mortgage”, which sounds even more unnatural and probably also incorrect?

I wouldnt say The reason why ... I would omit the why but apparently both versions are technically correct. I dont think D is wrong but Paying the morgage is the reason he sold his new car sounds better imo.

As in many of these examples, none of these is really correct or what would be used in real life. I would say something like "he had to sell his new car to pay the mortgage", or "he sold his new car to pay the mortgage".