The kid could not understand the book.
The kid (had not seen the book but) wouldn’t have understood.
Barnet ville ikke have kunnet forstå bogen.
Pronunciation of this Danish sentence, which is actually quite frequent, will in worst case be something like this:
\\Barneð vi egg ha ku forstå bogen\\
Having observed that it is quite understandable that Danish pupils make many spelling mistakes like this
Barnet ville ikke have kunne forstå bogen.
However, from a grammatical point of view this is a pity because it signifies lack of meaning. The sentence becomes a “sound picture” and not a dynamic construction.
I know – I know, you can ask “so what”, and “what is the big difference?”
The difference is that the ability to construct new sentences – hypothetical, conjunctive and so on, is lost. When reading a message full of errors one gets tired and somehow lose track of the meaning.