写过去的时候，是不是都要用过去式

这是不一定的。举个例子。下文中第一个空格是正确形式；第二个空格是提供的单词。
Peter is a middle school student.he is a kind boy.he( is )(be)always ready to help others.one afternoons,he(was leaving)(leave)school when it (began)(begin)to rain.he (saw)(see)an old man walking slowly in front of him.the old man(didn't have)(not leave)an umbrellla with him,and he(was)(be)wet all over.Peter went up to him and(held)(hold)the umbrellla for him.soon,they(got)(get)to the old man'home.the old man was very pleased and(said)(say),"it's very kind of you.thank you very much."with a smile ,peter said goodbye to him and(went)(go)away