Do not – Don’t
Cannot – Can’t
Are not – Aren’t
Would not – Wouldn’t
Will not- Won’t… but why is not Willn’t ?

β€œWill” comes from Old English willan (to want / to intend). Its negative form back then was nyllan (β€œnot-will”). Example: ic nelle = β€œI will not.” Over time, nyllan disappeared from common use. Instead, people just said β€œwill not.”

Contractions in English follow sound ease, not strict logic. β€œWilln’t” is hard to say (your tongue has to bounce awkwardly between the β€œl” and β€œn”). β€œWon’t” is smoother, so it won the battle of usage. Middle English had wyll not β†’ wynnot β†’ wonnot β†’ won’t. That β€œo” in won’t is basically a leftover vowel shift from the 15th–16th century. Think of it like β€œain’t” β€” not logical, but it stuck because it was easier to say.

So the short answer:

It was once closer to willn’t (like will not β†’ wynnot), but English speakers preferred smoother, faster sounds. That is why we got won’t.