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.