The broken plural is the one English has nothing quite like: instead of adding an ending, the word's own shape is “broken” and rebuilt on a new pattern — كِتَاب becomes كُتُب, رَجُل becomes رِجَال. There is no single rule; the patterns are learned together with the words, much as an English learner simply learns that “mouse” becomes “mice.”

From the free course The Language of Quran — Easier than English (Book 1) (LoQ1), taught by Ustad Muhammad Arjan Ali.