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!