> Autocorrect becomes an active enemy when I'm trying to type "I'd'nt've" or "y'couldn't'v'nt'd".

This must be the first time I'm on Autocorrect's side ;->

What is the "nt'd" in supposed to be short for?

"y'couldn't'v'nt'd" to me sounds like "you couldn't have unted", which is true as I've never "unted" anything in my life.

I guess "I'd'nt've" means "I don't have", but in British English we'd still write that as "I don' 'av'" (as we'd also drop the t).

I dunno man. I'm just typing what I hear & say. The first is "I do not think I would have done that", more or less. The second is "I could not have done that" but with an agreeing second negative? Like a hill people "I wouldn't do that if I weren't you."

Perhaps "I'd'nt've" is intended to be "I'dn't've", i.e. "I would not have":

I would -> I'd

would not -> wouldn't

So "I would not" -> "I'dn't"?

Sure, but I was asking about the "'nt'd" at the end of all that.

As a British English speaker, I can't even guess what verb they're trying to say that they wouldn't / couldn't have done.

Sorry, I should have made clear I was responding only to this part of your message:

  I guess "I'd'nt've" means "I don't have"

I think it meant 'I would not have', not "I don't have".