As far as I can ascertain, there are no rules for determining the correct suffix. It is a case of memorizing the correct one for the root word. There is a list of rules on the website Everything2, but as another webpage author comments, it would be less mental effort to simply memorize the words and spellings.
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EllycatI believe it stems from Latin.Sort of. But even attempts to relate these to the conjugations of Latin verbs end up failing. It's pretty much hopeless. You just have to look them up in a dictionary when you can't remember the correct spelling.