When the stem (or root) of a verb ends in a vowel (excluding "u" and "i") and the ending begins with a vowel, contraction of the vowels often takes place.

1. Verb stems must end in a, e, or o.
2. 'O' dominates over 'A' or 'E'; 'A' dominates over 'E.'
3. 'I' in the ending is never lost.
4. Omicron or epsilon, combined with omicron or epsilon, form a diphthong.
5. A diphthong remains a diphthong.
6. A long vowel remains a long vowel.

The stem must end in a, e, or o (the three vowels in the word 'CONTRACTED.' The vowels will dominate over each other in the order that they appear in the word 'contracted': thus 'O' sound over 'A' and 'E' and 'A' sound over 'E.'

Think of the two poles of a magnet. Two likes repel. So with Greek vowels and consonants. Often when two vowels or two consonants come together when an ending is added to the stem of a verb, vowel contraction or consonant modification is required.