Approximately 200 verbs are classified as irregular verbs and do not form the past tense and past participle the same way as regular verbs.  As a result, the principal parts of these verbs need to be memorized.

The majority of irregular verbs form the past and past participle by changing an internal vowel.
 

Some irregular verbs form the past and past participle by changing an internal vowel and adding -n or -en
to the past participle.
 

Other irregular verbs have the same form in the past and past participle or in all three principal parts.
 

If you are not sure about a verb's principal parts, check a dictionary.  If only the infinitive/present tense form is listed, it is a regular verb.  If the verb is irregular, the dictionary will list two or all three of the principal parts, depending on whether the past and past participle are the same.

The following is a list of some more of the common irregular verbs: