The past tense of "undo" is "undid." Just like many other regular English verbs, the past tense is formed by changing the "-o" ending to "-id." But let's dive a little deeper to understand why "undo" becomes "undid".
English verb tenses can seem confusing, but believe it or not, there's a method to the madness. When forming the past tense, verbs typically either take an "-ed" ending (like "walked" or "talked"), or they undergo a change in the vowel sound (like "run" to "ran").
In the case of "undo", it belongs to the latter group, known as irregular verbs. "Undo" comes from the prefix "un-" meaning not, and "do", so loosely speaking, "undo" means to 'not do'. Interestingly, its past tense "undid" follows the same pattern as of "do", whose past tense is "did".
The interesting thing about English is that while there are certain patterns and rules, there are also plenty of exceptions. And it's the exceptions, like "undid", that make learning English a bit more of a challenge -- but also much more fun!
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