couldn't do and can't have done/couldn't have done

[Saneta]

Where should I use :,,couldn't do'' and when : ,,can't have done''/,,couldn't have done''?
What is the difference between them?
Thank you very much.

[Alan]

Hi Saneta,

Let's follow these through:

I couldn't do it yesterday but I hope I can do it today.

I know I put the key on my desk just now and so I know I can't have lost it.

I know I put the key on my desk last night and so I couldn't have left it in the door.

Alan
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