Stem changing verbs: O>UE
One of the forms of stem-changing verbs is called O to UE. That's because the O in the stem of the verb changes to UE in all of the forms except vosotros and nosotros. Some folks call stem-changers "boot verbs" or "shoe verbs" because of the way they look on the verb chart. Use these resources to learn more.
Grammar explanation: Stem-changing o>ue verbs
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Stem changing verbs