any verb whose past tense and past participle are not formed by adding -ed, -d, or -t to the present tense, a verb that does not follow the general rules of inflection
Two months later, Theodora was reading the Anabasis, while Hubert was still toiling over the intricacies of the irregular verb.
The verb substantive is generally dealt with as an irregular verb.
The irregular verb sum is so important for the conjugation of all other verbs that its inflection is given at the outset.
But none of them left their sticks to help you through the irregular verb?