can't get your head around sth

I just can't get my head around why he made such a decision.

我就是无法理解他为什么会做出这样的决定。

I can't get my head around this complex theory at all.

我完全无法理解这个复杂的理论。

She can't get her head around the concept of calculus in math.

她无法理解数学中的微积分概念。