表达 “make a mental note” 的意思是 “试着记住一件事情”。如果想说明要记在脑子里的事情具体是什么，可以说 “make a mental note of something”。

例句

I don't have a pen and paper at the moment, so I'll make a mental note of it.

我现在没有纸和笔，所以我会记在心里。

I'll make a mental note of the date of the wedding.

我会在脑子里记下婚礼的日期。

You'll need to make a mental note of what she says in the meeting.

你需要在心里记下她在会议上说的话。