"Irregular verbs are common verbs in the English language that do not follow the simple system of adding “d” or “ed” to the end of the word to form the past tense. That means the spellings can be a little tricky. Some irregular verbs follow patterns, such as drink–drank, spring–sprang, know-knew, and blow-blew but you can’t use those patterns with predictability. Unfortunately, learning irregular verbs means memorization."

http://www.k12reader.com/irregular-verbs/