wouldn't've

A compound contraction for "would not have". Also see couldn't've and shouldn't've.

I was going to explain the whole thing to him, but he wouldn't've understood.

couldn't've

compound contraction of "could not have"

She couldn't've done it, she's way too stupid for a plan that complex! Also see wouldn't've and shouldn't've.

shouldn't've

Compound contraction of "should not have". Also see wouldn't've and couldn't've.

You shouldn't've said that, now Grandma's really mad!