In the English language, all verbs have different forms or tenses. These can include present tense, simple past tense, and past participle. Irregular verbs, also known as strong verbs, are deemed irregular because they do not follow the system of adding -d, -ed, or -ied at the end to form the past tense. No pattern of spelling an irregular verb in past tense is predictable, which means these spellings must be memorized.