I always had the feeling myself that there has to be some other way than learning 6 principal parts for every verb (or 7, as Emma shows us!

)...<br /><br />Basically, what you're seeing when [face=SPIonic]
pe/mpw[/face] becomes [face=SPIonic]
pe/pomfa[/face] is a change in vowel grade. Greek morphology shows something called ablaut, where you have a root (I'm not sure if 'root' is the correct term), and by inserting different vowels between the letters of this root you get different types of words. So, for instance, if we take the root [face=SPIonic]
klp[/face], we have:<br /><br />e-grade: [face=SPIonic]
kle/ptw[/face] (
verb: to steal - note how a great many verbs show the e-grade...)<br />o-grade: [face=SPIonic]
kloph/[/face] (
noun: theft - there are a number nouns representing an object related to the verb in this manner which also show the o-grade)<br /><br />(There is also a 0-grade (zero grade - no vowel, or no [face=SPIonic]
e[/face] or [face=SPIonic]
o[/face] at any rate), but I can't think of a word from [face=SPIonic]
klp[/face] which shows it, or any pattern in meaning about it...)<br /><br />Mind you, I'm a beginner myself and not entirely sure about this concept of ablaut - I really would rather have someone more knowledgeable explain it properly!<br /><br />Sadly, the appearance of the different vowel grades isn't really predictable - it's more something you notice 'after the fact' - but since there are some patterns connected with them, knowing about them can help in learning vocabulary.<br /><br />So: I don't know about any hard-and-fast rules which would alert you when to expect a change in vowel grade, but do be aware that [face=SPIonic]
pe/mpw[/face] is one of many verbs to have a present ind. act. showing the e-grade but a perfect ind. act. showing the o-grade...<br /><br />I hope that is more helpful than confusing! :-X