Here's why:
* Regular verbs form their past tense and past participle by adding "-ed" to the base form. For example, "walk" becomes "walked".
* Irregular verbs have different, often unpredictable, forms for their past tense and past participle.
The past tense of "teach" is "taught", and the past participle is also "taught". This doesn't follow the "-ed" pattern, making it irregular.