Jack & Jill ~ "Could Have" vs "Couldn't Have"

Jack: I "could" have done it.

Jill: If you "could have", why didn't you?

Jack: Something stopped me.

Jill: That means you "couldn't have" done it even if you wanted to.

Jack: I "could have" but there were to many obstacles then.

Jill: It's because of those obstacles that you "couldn't have" done it.

Jack: If I knew then what I know now I "could have" done it.

Jill: 

If you were back there now, you still 

wouldn't know what you know now, 

so you still "couldn't have" done it. 

Jack: I think... I just broke my crown.

===.