Basic contractions like “could’ve” and “should’ve” are often misspelled because the pronunciation sounds more like two words than one.
This is one you’ll need to memorize, but understanding the basic formation of contractions helps. A contraction is the joining of two separate words by dropping a letter (or two) and replacing it with an apostrophe.
DO NOT = DON’T (drop the O)
WOULD HAVE =WOULD’VE (drop the H and A)
from Grammarly Blog https://www.grammarly.com/blog/2016/learn-the-trick-for-spelling-couldve/