As the title says. An "O" before the last syllable (-er or -ir) is often turned into "ue" when conjugating, e.g. puede, duerme, vuele.
Verbs ending in -ar don't: nota, vota, ...
But poner, comer, etc. ends in -er, so why aren't they puene, cueme?
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The verbs that alternate between o and ue (like the verbs that alternate between e and ie/u) are the result of a phonetic change that happened to the short /o/ and /e/ in Latin as it transitioned to modern languages (the vowels in Latin could be either long or short).
For the most part, if you know the Latin word has a short vowel, you can predict that the change will happen, although it's not a perfect system (correr or responder should change, but don't, at least not in Spanish).