Difference between “could do” and “could have done”

Question

I tried to find the difference between following two sentences:

I could do it.

I could have done it.

I am still confused between these two sentences. Please let me know what is the difference between them in context.

Answer 1

John: We are having a party tomorrow. I need someone to cook the fish.

Alice: I could do it.

That is, in the future, Alice can cook the fish.

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John: The party was a disaster. No-one knew how to cook the fish.

Alice: I could have done it.

That is, in the past, Alice could have cooked the fish.

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Compare to:

John: We are hungry. We need someone to cook the fish.

Alice: I can do it.

That is, in the present, Alice can cook the fish.