There are some irregular plurals in English (child/children, goose/geese), but all of the ones I know of share the same root word. In some languages, there are some irregular pairs where the singular ...
In English, there are three suppletive adjectives: good, bad and far. Their comparative and superlative forms derive from different stems, i.e., we have best instead of *goodest, worse instead of ...
The verbs be and go have the nice peculiarity that their various forms (be/was and go/went) come from originally distinct verbs. Are there other such verbs?