A verb that does not form its Past tense and Past participle in a regular way. Regular verbs work like this:
Stem Past Tense Past Participle
happen happened happened
tango tangoed tangoed
smile smiled smiled
If the stem ends in a consonant or a vowel other than e, then you add the letters ed. If the stem ends in e then the letter d is added.
Irregular verbs do not follow this pattern. Linguists divide them into seven different groups within which there are patterns. For everyday purpose it is more helpful to divide them into three:
 Verbs in which the stem, the past tense, and the past participle each have a different form.
 Verbs in which the past tense and the past participle have the same form which is different from the stem.
 Verbs where all three are the same.
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