Irregular verbs don't follow the rules.
What is an Irregular Verb?
A regular verb follows rules for how it looks in a specific form. For example, all regular verbs have an -ed on the end in the simple past tense form. (look/looked, study/studied, shop/shopped.)
An irregular verb is a verb that doesn't follow the rules for how it looks in the simple present tense, past tense and/or the past participle. Most irregular verbs are irregular in two tenses: the simple past, and the past participle.
Only 2 verbs are irregular in the simple present tense in both spelling and pronunciation. They are: to be and to have. To do and to say are regular in spelling, but irregular in pronunciation.
Present tense irregular verbs (both spelling and pronunciation):