This is off topic and I hope that you take it in the spirit offered. I can never decide if it's rude or helpful but the intent is to be helpful.
Teach is one of a handful of words in English that doesn't follow the normal rules for it's past tense form. The past tense for teach is taught. I honestly can't tell you how to identify which words use a different form for their past tense. I think it's just something that you learn through experience.
https://www.esl-lounge.com/reference/grammar-reference-most-common-irregular-verb-list.php