The principal parts of a verb are the four forms of the verb from which all forms of the verb can be made. In English the four principal parts are the present (or infinitive), the past tense, the past participle, and the present participle.
|Infinitive||Past||Past Particple||Present Participle|
To walk is considered a regular verb because we add a -d or -ed to the verb for the past and past participle.
To run is considered an irregular verb since one or more of the principal parts is formed in a nonstandard way.
Because the present participle is always formed the same way by adding -ing to the infinitive, some lists of principal parts omit it.
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