Here is the sentence:

Unaware that his alarm clock had not rung at 6 a.m., Levi slept under the warm blankets while Dr. Ribley tought the semester's most difficult algebra concept to the students who had arrived on time.

The third verb, tought, is the problem. As you know, this sentence requires the simple past. Teach is an irregular verb; taught is the correctly spelled form.

Verb Simple Present Simple Past Past Participle
teach teach(es) taught taught

Go to the next sentence, champ!

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