Here’s a teaser from Anatoly Liberman’s short blog post, which contains a number of other interesting answers to vexing problems in English grammar.
Very long ago, one of our correspondents asked me how irregular forms like good—better and go—went originated. Not only was he aware of the linguistic side of the problem but he also knew the technical term for this phenomenon, namely “suppletion.” One cannot say the simplest sentence in English without running into suppletive forms. Consider the conjugation of the verb to be: am, is, are. Why is the list so diverse? And why is it mad—madder and rude—ruder, but bad—worse and good—better? Having received the question, I realized that, although I can produce an inventory of suppletive forms in a dozen languages and know the etymology of some of them, I am unable to give a general reason for their existence. I consulted numerous books on the history of the Indo-European languages and all kinds of “introductions” and discovered to my surprise that all of them enumerate the forms but never go to the beginning of time. I also turned to some of my colleagues for help and came home none the wiser. So I left the query on the proverbial back burner but did not forget it. One day, while feeding my insatiable bibliography and leafing through the entire set of a journal called Glotta (it is devoted to Greek and Latin philology), I found a useful article on suppletion in Classical Greek. Naturally, there were references to earlier works in it. I followed the thread and am now ready to say something about the subject.