* Regular verbs form their past tense and past participle by adding "-ed" or "-d". Examples: walked, played, helped.
* Irregular verbs have unique forms for the past tense and past participle that don't follow the "-ed" or "-d" pattern. Examples: go -> went -> gone, see -> saw -> seen, do -> did -> done.
Since "shown" doesn't follow the regular pattern, it is considered an irregular verb.