Verbs are divided according to their forms into regular and irregular verbs.
A regular verb forms its past tense form and past participle form by adding d or ed to the present tense form. In all cases, with regular verbs, the past tense form and the past participle form are the same.

Regular Verbs

Present	Past	Past Participle
love	loved	loved
start	started	started
learn	learned	learned

An irregular verb does not form its past tense form and past participle form by the addition of ed to the present tense. In some cases with irregular verbs, the past tense form and the past participle form are the same; however, in many cases, they are not the same.

Some Irregular Verbs

Present	Past	Past Participle
see	saw	seen
go	went	gone
write	wrote	written

Most of the most common verbs in English usage are irregular verbs in part because historically they have been in the language for the longest time. 
An auxiliary verb is a verb that is used in conjunction with other verbs to form verb phrases that are the more complex verb tenses. Forms of the verb to be, to have, and to do as well as may, can, should, and shall are the most important auxiliary verbs.

At this point proceed to Exercise 18.