6a. The four principal parts of a verb are the infinitive, the present participle, the past, and the past participle.
Singular subjects take singular verbs.
Plural subjects take plural verbs.
6b. A verb that forms its past and past participle forms by adding -d or -ed to the first principal part (Infinitive) is a regular verb.
6c. A verb that forms its past and past participle in some other way than a regular verb is an irregular verb.
Irregular verbs form their past and past participle in various ways: by changing the vowel, by changing consonants, by adding -en, or by making no change at all.
Special problems: lie/lay, sit/set, rise/raise
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