Help - 忽然 and 突然 differences

Hi Everyone,

I could not find this question on ChinesePod, so sorry if it's already been addressed.  But I need help understanding the difference between 突然 and 忽然.  I'm sure there is a difference, but all info I've found does not give me a clear answer.  Hoping someone can tell me very clearly these two...  In my Chinese book one of the exercises is to choose from 突然 and 忽然。

例：

1.真奇怪，刚才还好好的，怎么______就哭了？

2.这事情太_____了，我不知道该怎么办。

3.______来了一阵大风，把窗户吹开了。

4.谁也没想到会发生这么_____的事。

Any help is greatly appreciated.