What is the appropriate use of the modal 'Could' in this example?

Question

(Question moved from English Language & Usage.)

I was writing a passage of dialogue wherein one of the characters reflects on their ability to not do something in the past. See a similar example below:

[1] They were making a mockery of his name. I couldn't say nothing.

After this line, the other person in the conversation expresses their contrary opinion.

[2] Yes, you could've.

As you can see, the speaker in Example 1 uses the modal 'could' to express their capability (or lack thereof) in the context of an action in the past — that is, saying nothing after the mockery. The speaker in Example 2, however, uses 'could' with the perfect auxiliary 'have' to refer to the exact same event, disregarding the fact that they are having the conversation in the same time.

I feel as though this is correct, as the first person is speaking from the point of view of the past, the same way that an author would write a past-tense story, whereas the second is immersed in the present, so they feel the need to use the perfect aspect.

Is there any requirement to change either of these two examples? For example, would you change Example 1 to 'couldn't have said nothing'? Conversely, would you change Example 2 to 'you could'?

Answer 1

I think it's fine as it is - as far as the second person is concerned it is a hypothetical. But Yes you could is probably a more natural reply.

[Note that for many English speakers I couldn't say nothing normally means I couldn't say anything unless the nothing is strongly emphasised. If your character is already established as somebody who doesn't use so-called "double negatives", that'll work; but reading your example without that context I at first thought that you had mastered that bit of English dialect, and you intended it to mean "I couldn't say anything".]