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- If the part wasn't the reason to cause the spaceship to explode, what else could have done?

I made this sentence and wrote "could have done" instead of "could have caused" to avoid repetition, but since "to cause" isn't the main verb in "if clause" and is to-infinitive, I'm not sure whether it's grammatically okay to use "done" to avoid repetition.

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If that component didn't cause the spaceship to explode, what else could have done it?

If that component didn't cause the spaceship to explode, what else could it have been?

fire1the reason to cause

No. 'reasons' don't 'cause' things.

CJ

Comments  
fire1- If the part wasn't the reason to cause the spaceship to explode, what else could have done?

Better: It is determined that defective components could have been the cause of the spaceship explosion. Nothing else!

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 CalifJim's reply was promoted to an answer.
CalifJim

If that component didn't cause the spaceship to explode, what else could have done it?

If that component didn't cause the spaceship to explode, what else could it have been?

fire1the reason to cause

No. 'reasons' don't 'cause' things.

CJ

- 2 l. A doctor told me to cut back on junk food for my health, but I didn't/couldn't, because I liked it so much in the past.

Then, is it possible to omit "cut back" in "I didn't/couldn't (cut back on it) as in the sentence above even though "to cut back" isn't the main verb in the first clause?

I'm not sure "did" can be used to replace the infinitive "cut back" as in sentence 2.

fire1

- 2 l. A doctor told me to cut back on junk food for my health, but I didn't/couldn't [no comma before a because-clause] because I liked it so much in the past.

Then, is it possible to omit "cut back" in "I didn't/couldn't (cut back on it) as in the sentence above even though "to cut back" isn't the main verb in the first clause?
...

Yes. do is a "pro-verb"; it can stand in place of a verb. Modal verbs can do the same thing. So both "didn't" and "couldn't" are OK in your sentence.

It doesn't matter that "cut back" isn't the main verb in the first clause because you have "me to cut back" and "I didn't/couldn't". "me" matches "I". The doctor isn't mentioned again at all.

CJ

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