Why does this contraction sound wrong?

My cousin wrote the following today:

Never would I've believed.

The use of "I've" here, rather than "I have", sounds extremely strange to me. Is it actually incorrect? If so, is there some sort of rule I could use to explain what makes it so?

Solution 1:

Never would I've believed.

The reason I've sounds wrong here is that I've is a contraction of I have, where the have is in the present tense, whereas the have in the example should be infinitive as it follows would.

Note the uninverted version is one of these:

I never would have believed.

I would never have believed.

I would have never believed.