It's beyond me. 我完全不能理解。

It's over my head. 太难懂了。

I don't have any idea what's going on. 我压根不知道是怎么回事。

I'm not sure what you mean. 我不明白你的意思。

I can't see your point. 我不明白你在说什么。

That's not clear. 我不清楚。

I don't really understand. 我不太明白。

I'm confused. 我有点晕。

I don't get it. 我不懂。

I'm not following. 我有点不明白了。

I don't understand. 我不明白。