The American Heritage Book of English Usage. Columbia Encyclopedia World Factbook American Heritage Coll. A Practical and Authoritative Guide to Contemporary English. The principal parts are used to form tenses.
Grammars usually classify verbs as regular and irregular.
Regular verbs form their past tense and past participle by adding the suffix -ed to the base form. They have plodded, She tried, We had tried, and so on. As these examples show, the spelling of these forms sometimes involves modification of the base form. The rules for spelling the principal parts of regular verbs are discussed under Word Formation. 1 Irregular verbs do not follow the -ed pattern of regular verbs.