Supposing you _________ the lottery, what would you do with

一个大路问题：如果你赢了500万,准备拿钱去干嘛?
所以在对方问你这个问题的时候,是一种对“一般情况”的假设,所以虚拟语气要用一般时态的过去式.

如果对方的问题是：上个月的乐透如果你中了,你准备拿钱去干嘛?
那你就选对了.
虚拟语气是高中英语语法的一个难点,教科书、网上、课外资料里面的讲解资料多不胜数,你可以自己去翻.我在这里可以给你非常简单地归纳为三种用法：

对过去的假设——所谓过去,当然是已经发生的事（如果细分,这里这个过去发生的事也要分为两种情况-过去发生不延续的动作,以及过去发生延续到现在的动作,具体看教科书吧）,而你去假设的事情当然就成了“过去的过去”,所以要用 had done sth.例句：If you hadn't married her,what would you be doing now?
对一般情况的假设——如上面所说.例句：If I were a teacher,I would never force boys and girls to sit seperately.
对未来的假设——用词跟第二种情况一样,只是意思略有不同.例句：If I were to retire before 40 years old,I would have to work 2 times as hard.