Some verbs form their past tense by merely changing the vowel in the body of the present without adding any ending. Such verbs are called strong verbs. It may be noted that these verbs form their past tense by merely changing the inside vowel of the present tense without having ED or D or T added to the present. Such verbs are called strong verbs as they are able to make their past tense without having anything added.
Note how the past tenses of the following verbs are formed.