关于过去进行时 当然可以换成别的时态，语法上是没错，但意思就有出入了was painting-是正在画的painted，had painted-是画好了的
Are you sure 的完整肯定回答是什么 Yes，I am sure.Are you sure？你肯定吗；你确定要…例句：1.Are you sure you heard right？你确定你听见的是正确的吗？2.Are you sure about the name of that hotel？关于。
have to有没有过去分词形式 have to 是个词组。意思是must，属于情态动词，表示过去用had to，不存在过分形式。Of course，it had to start raining as soon as we got to the beach.可恶的雨，我们一到海滩它就非得下起来。过去时There has to be a reason for his strange behaviour.他的古怪行为一定事出有因。现在时
