词性不同。
midnight：
n. 午夜，半夜
adj. 半夜的，午夜的
例句：
It was going on (for) midnight.
快半夜了。
mid-night：
n. 午夜，半夜
A fire broke out in the hotel in the mid-night.
这个酒店半夜发生了一场大火。
A fire broke out in the hospital in the mid-night.
一场大火在午夜时分发生在医院。
The one and other heart waits for you calling at the mid-night.
一个又一个温心的夜里，等待着你的呼唤。