Irregular Verbs

Irregular verbs present a great challenge for non-native speakers of English. Regular verbs would add an -ed for both the simple past and the past participle. For instance, he cooks becomes he cooked or he has cooked. But many common verbs defy these rules, that is, are irregular. For example, eat becomes ate (past) or eaten (past participle).

He eats his breakfast every morning.
He ate breakfast yesterday.
He has already eaten breakfast today.

How is the non-native speaker supposed to know what to do? There are lists of irregular verbs online, though one must consult a dictionary
to be sure that there is not also a regular conjugation of the same verb. For example, to hang is usually conjugated with hung (past and past participle) in these lists, but if you are talking about hanging a person, the conjugation is regular, that is, hanged.

They hung the picture on the wall.
They hanged the traitor from the tallest tree they could find.

Some verbs also have both an irregular and a regular form of the past participle, but their meaning does not change.

He has sewed patches on the sails.
She has sewn two new blouses.

Three lists that I find useful are those offered by PBS, the Purdue University Writing Lab and Susan Jones (via Georgia State University), who also offers this general explanation of English verbs.