ἀπόλλυμαι I'm lost

 

Stem: ἀπολλυ. The endings are attached directly to the stem.

Whereas the stem of βούλομαι is βουλ, if we attached the endings directly to the stem we'd have

βούλμαι, βούλσαι, βούλται,βούλμεθα,βούλσθε,βούλνται.
Some of those forms would be a little difficult to say. That is why a vowel was inserted, placed between the stem and the ending.This vowel is called called thematic, from the Greek verb τίθημι, I put, place.

Compare:

 Present 
wants to Who? is lost, destroyed
βούλ ομαι  I   ἀπόλλυ μαι 
βούλ ει/ῃ  you (sg) ἀπόλλυ σαι 
βούλ εται     he, she, it    ἀπόλλυ ται 
βουλ όμεθα  we ἀπολλ μεθα 
βούλ εσθε  you (pl) ἀπόλλυ σθε 
βούλ ονται  they ἀπόλλυ νται