Principal parts refer to the fundamental forms of a verb, which are essential to express different tenses and moods in a sentence. There are various synonyms to describe these principal parts, including base, root, stem, infinitive, present participle, past participle, gerund, and past tense. The base or root form typically describes the simple present tense, whereas the past tense emphasizes the action's completion in the past. The present participle emphasizes the continuity of the action in the present, and the past participle describes the completion of the action in the past. The gerund is a verbal noun that ends with -ing, while the infinitive signifies the verb form preceded by 'to.' These synonyms assist in understanding the grammatical structure of verbs and their role in sentence formation.