What is an irregular verb?

An irregular verb is a verb which does not follow the rules for regular verbs.
The rules for regular verbs in the past are the following:

We normally add -ed to the verbs

e.g. work worked worked

If the verb ends in "e" we only add -d

e.g. love loved loved

If the verb ends in consonant + y, we remove the "y" and add -ied

e.g. study studied studied

With verbs of 1 syllable which end in consonant + vowel + consonant, we double the last letter.

e.g. stop stopped stopped

Note: For full information about verbs with two syllables please consult a book of grammar.