what+if引导的条件从句是否可以用在虚拟语气中?What would you do if you had a million dollars?这句话可否改为:What if you had a million dollars?

不可以.
What if you had a million dollars?这句话是个虚拟语气引导的句子，与实际情况不符。意思是“假如你有100万的话呢？”
What would you do if you had a million dollars?这句话只是一个条件句，是对将来要做某事的提问。意思是“如果你有1 million ，你将用它来干什么？
两者引起的上下文意思是截然不同的。

不可以.
What if = What would happen if? 要是...又怎样?它引导的一般是真实条件句(事情有可能发生),比如:
What if it rains when we can't get under shelter?假若下起雨来,我们又没处避雨可怎么办?
What if the rumour is true? 万一谣传真有其事呢?
而虚拟语气情况下,事情是不可能发生的,就比如,If I were you, I would take his advice. 但事实上我不可能是你.

不可以!改后的意思与原句不同了.what if 这个词组表示“如果……会怎样”,用来表示假设（也就是虚拟语气）、表达感慨.what if you had a million dollars 可以这样译：“如果你有1million,那么你现在的境况会发生怎...

可以