Why find found, bind bound, fight fought, wind wound (i / ou) but cling clung, dig dug, spin spun, sting stung (i /u)? [closed]

Solution 1:

Old English ind, und were lengthened to [aɪnd], [aʊnd]

The spelling “ou” is used in present-day English found, bound, wound to represent the diphthong vowel sound [aʊ]. Before the consonant cluster "nd", [aʊ] developed from Old English [u] (through a lengthened stage [uː]). But [aʊ] did not develop before “ng” or “nn” (the "n" in spun is a simplification of original "nn"). So Old English und generally became modern English “ound”, but Old English ung and unn did not become modern English “oung” or “ounn”.

Old English [i] was also lengthened and diphthongized to [aɪ] before “nd” (in find, bind, wind v.), but this is not associated with a spelling change.

Old English h affected the pronunciation of a preceding vowel

Fight/fought comes from an Old English verb containing h. Old English h also caused changes in the pronunciation of a preceding vowel. The development of vowels before h is too complicated for me to summarize, but in modern English spelling, "ugh" is not normally used by itself: instead, you'll see it only in combination with a preceding vowel letter as in "ough" or "augh". Likewise, "egh" does not generally occur: only "igh", "eigh" and "aigh".

Dig/dug has a different history

Verbs with “i” in the present and “(o)u” or “a” in the past or past participle generally go back to the Old English class 3 strong verbs.

The verbs from your list that were class 3 in Old English: find, bind, wind, fight, cling, spin, sting.

Dig does not go back to an Old English strong verb; its past tense form dug was created by analogy (according to the Oxford English Dictionary, in the 16th century).