Here's why:
* Regular verbs follow a consistent pattern for their past tense and past participle forms (e.g., "walk", "walked", "walked").
* Irregular verbs have unique, unpredictable forms that don't follow this pattern.
In this case:
* Present tense: speak
* Past tense: spoke
* Past participle: spoken
The past tense "spoke" and the past participle "spoken" don't follow the regular "-ed" ending pattern.