When I was going to school, I met my friend.
When we were going to the cinema, we met our grandmother.
When grandmother was going home, she saw many children in the yard.
When Henry was walking in the forest, he found a bear cub.
When we were walking in the forest, we saw a hare.
When I was washing the floor, I found my old toy under the sofa.
When granny was reading a book on the sofa, she fell asleep.
When I was playing in the yard, I suddenly saw my old friend.
When Nick was running about in the yard, he fell.
Все предложения посторены по одной конструкции - в придаточном предложении (оно начинается с союза when) используем Past Continuous - действие длилось, продолжалось, глагол отвечает на вопрос "что делал(а),
а в главной части предложения глагол ставится в Past Simple - действие моментальное, глагол отвечает на вопрос "что сделал(а).