The Morphology of Biblical Greek, by William D. Mounce

If you can’t figure out why a Greek word is changing spelling in strange ways, then the first resource you should go to is Bill Mounce’s Morphology of Biblical Greek. Today’s video also overviews A Morphology of New Testament Greek, by Brooks & Winberry
Morphology of Biblical Greek - amzn.to/2ZoBDRO
Morphology of New Testament Greek - amzn.to/3pypp3O