Anyone using Croy will find that, when future and aorist tenses are introduced, stems are usually given only for the most common verbs. A number of verbs seen in Croy do not have their principal parts explicitly given, even in Croy's glossary. I think this is a huge flaw in the book, since memorization of principal parts is encouraged in every textbook. Anyone wanting to learn the stems systematically when new forms are introduced will need to use another resource.
Luckily, Laura Gibbs' online Koine course includes pages that lists all verbs (by chapter), with four principal parts (if it applies to the verb). You can find them here:
This post (like several of mine) is probably for forum lurkers and people that may later search the forums looking for support with particular textbooks.