Question

one million dollar 还是one million dollars . 我有一百万

your decision can change their lives 还是your decision could change their lives .
未发生应该用哪个？原因

Answer 1

dollar 是可数名词，复数要加S, 所以 one million dollars 是对的。
可能发生的假设句，用一般现在时来表示。不可能发生的假设句用过去时表示。
这里是可能发生的假设句，所以用 can.
例句：
可能发生的：If I am free this afternoon, I will go to shopping with you.
不可能发生的：If I were you, I would never do it.

Answer 2

一百万美金，当然要加s. one million dollars. 是could