Here's why:
* Regular verbs follow a predictable pattern for their past tense and past participle forms (usually adding "-ed").
* Irregular verbs have unique, unpredictable forms.
Let's look at the forms of "ascend":
* Present Tense: ascend
* Past Tense: ascended
* Past Participle: ascended
Notice how the past tense and past participle don't follow the regular "-ed" pattern. This makes "ascend" an irregular verb.