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I have following sentence:

If Jonas Salk hadn’t discovered a cure for polio in 1952, millions of people couldn’t have been spared from this disease

My teacher said that "couldn’t" sounds bad here and I need to use "wouldn’t", but, unfortunately, she wasn't able to argue that well enough. Can somebody explain me, please, why do I need to use "couldn’t" here?

  • Both are right. Could have been and would have been do not mean the same thing exactly. Millions of people would not have been saved because they could not have been saved. Tell her that...or him. – Lambie Dec 19 '16 at 22:03
  • Both sound odd. The negative form is a poor choice here. "... millions of people would/could have succumbed to this disease." would be preferable to me and, of the two, "would" is preferred. – Catija Dec 19 '16 at 22:48
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When stating a hypothetical situation, "would" is most commonly used.

If I had a girlfriend, I would take her out dancing every weekend.

It isn't wrong to say something like:

"If I had a girlfriend, I could take her out dancing every weekend."

But these are two different statements. The first says "if A is true, then I will do B." The second says something more like, "If A is true, then I have the opportunity or ability to do B, but I may or may not do so."

When talking about past situations like the polio vaccine where something's ability or capacity is known because it happened, "could" doesn't make sense. It is true that Salk's vaccine has prevented millions or even billions from being stricken by the disease. So, since you have set up a hypothetical, use "would" to complete the statement.

If Salk had not invented his vaccine, many more would have suffered from polio.

It is possible to use "could" but only in cases where, for example, one past event relied on another past event:

If the Wright Brothers had not invented the airplane, modern transcontinental travel could never have been possible.

But even here, "would" works just as well.

  • "It is possible to use "could" but only in cases where, for example, one past event relied on another past event" - well, I guess, this is my case, isn't it? And I can use "couldn't" in my sentence, right? – Nikolai Mavrenkov Dec 20 '16 at 10:26
  • Not necessarily. It just depends on what you want to say: The boys couldn't have caused trouble if they had not gone out that night. – Lambie Dec 20 '16 at 15:10
  • @NikolaiMavrenkov Yes, which is why I think your use of "couldn't" isn't wrong. It depends on what you want to say. But, since you phrase your sentence as a hypothetical, "wouldn't" is more natural than "couldn't". – Andrew Dec 20 '16 at 15:20

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