Ablauting

PIE nouns, adjectives, and verbs can be divided into two basic groups based on their inflectional patterns: those that had an ablauting short vowel, e or o (in shorthand -e/o-), directly before the inflectional endings (the case-endings in nouns or adjectives, the personal endings in verbs), and those with no such vowel.

As an irregular verb, however, its past tense form is characterized by ablauting ('any vowel change that alternates') in which /e/ -> /o/ (/brek/ -> /brok/) and its past participle form is characterized by both ablauting and /(e)n/ addition.