@MontereyJack,
Quote:In its simplest form, "I can do it today".
"I could do it yesterday, but I strained a muscle". You will notice you can't say, "I can do it yesterday".
There's no syntactic connection between your examples of CAN & COULD, MJ. There is a semantic connection. That semantic connection in your example is 'could' describing a past general ability, not 'could' acting as the past tense of CAN.
A: [to B] I can do it today.
[Three hours later]
B: Is it done?
A: Yes, it's done. *I could do it today.*
[* denotes ungrammatical for the situation]
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Quote: "I could do it yesterday, but I strained a muscle". You will notice you can't say, "I can do it yesterday".
That's simply because that remains one of the "jobs" that COULD has retained in modern day English.
Notice that COULD can be used in situations to describe present and future events.
"I could do it yesterday, but I strained a muscle so I'm afraid that I couldn't do it for you even now."
Quote: "I could do it yesterday, but I strained a muscle". You will notice you can't say, "I can do it yesterday".
But you will notice that you can say,
"He can't have done it yesterday!"
What is that CAN operating as the past tense of?