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Are the following sentences natural?

John said he would help Mary, but I could not be sure when he would come back.

John said he would help Mary, but I was not sure when he would come back.

What difference, if any, is there between "was not sure" and "could not be sure"?

I'd appreciate your help.

  • What's the time frame? Each is a slightly different perspective on when the statement and the return happen. – Andrew Jul 22 '18 at 5:43
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The version with could expresses past capability (can -> could), here negated. It was not possible for you to be sure. There is an implied hindrance or impediment to you being certain.

The version with was expresses a simpler past fact: you were not sure.

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