Mnemônicos para memorizar Verbos Irregulares.

One mnemonics to one of the irregular verb cases is:

-ought or -aught?

Mnemonic valid for Explanation Examples
irregular verbs
If -a is in the infinitive of the verb it is also used in the Simple Past and the past participle.
teach - taught
catch - caught

buy - bought

http://www.englisch-hilfen.de/en/words/mnemonics.htm


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As for other irregular verbs you have to be acquainted to them, there is always a list of them at the end of Brazilian English textbooks. This is the kind of topic to wich the saying "the practive makes perfect'' proves true, the more you read novels, or even train them with textbooks exercises the more you will get used to those irregular verbs.

There are plenty of sites with them as well. The likes of EMB (English Made in Brazil) etc.
http://www.sk.com.br/sk-irrve.html