I cannot understand (that)…

I cannot understand why you made such a silly decision.

我不明白你为什么作出这样一个愚蠢的决定。

I cannot understand why you care for her so much.

我不明白你为什么那么在乎她。

I cannot understand how you could overlook such an important man.

我不明白你为什么会忽略这么重要的一个人物。

例句：

I cannot understand why you left without telling any of your friends.

我不明白你为什么不跟你任何一个朋友说一声就离开了。

I cannot understand why he betrayed his country.

我不明白他为什么要叛国。