While John was riding to school, an accident happened. When John was riding to school, an accident happened.

While my parents were working on the farm, they heard a very loud strange noise. When my parents were working on the farm, they heard a very loud strange noise.

She saw an alien. She was eating sushi. When she was eating sushi, she saw an alien.

While he was having lunch, they arrived. When he was having lunch, they arrived.

I was watching TV at 8 o'clock yesterday. Suddenly, the doorbell rang. When the doorbell rang, I was watching TV at 8 o'clock yesterday.

I was walking home. I saw an object like a UFO in the sky. I was walking home when I saw an object like a UFO in the sky.

Having completed the test, John handed it in and left. When John had completed the test, he handed it in and left.

She's going to have a talk show on the weekend. (Câu này không yêu cầu điền "When", nên mình giữ nguyên câu như vậy)