An irregular verb is one that does not follow the standard progression for various forms. Regular verbs are treated as shown below:
Simple present: talk (I talk.)
Simple past: talked (I talked.)
Present participle: talking (I am talking.)
Past participle: talked (I had talked.)
Infinitive: to talk (I like to talk.)
Irregular verbs take the same form in three of these categories, but their simple-past and past-participle forms are irregular, and there lies the difficulty.
However, notice a common feature among verbs with the long i sound:
Simple present: ride, drive, write
Simple past: rode, drove, wrote
Past participle: had ridden, had driven, had written
In the simple past, the vowel changes to a long o sound, while the vowel becomes a short i sound in the past-participle form. Therefore, you can extrapolate for the progression from simple present to simple past to past participle for other words, as with rise, rose, had risen.
For many verbs with vowels that sound like a long e or a short i, the progression is as follows:
Simple present: drink, sink, sing
Simple past: drank, sank, sang
Past participle: had drunk, had sunk, had sung
From this pattern you can deduce that shrink will change to shrank or had shrunk, while begin progresses to began and had begun and swim changes to swam and had swum.
Of course, there are many exceptions: Bind progresses to bound, not bond, and to had bound, rather than had binden. Likewise, the progression from blow to blew to had blown is not matched by show, showed, and had shown. When in doubt, search online for a list of irregular verbs to note the correct forms for your problem verb.