There are many troubles and problems in learning a language. We talked about the a logic trap on 何だ?!?, but perhaps the most difficult trap of them all is the Unknown Unknowns. Donald Rumsfeld won an award for putting his foot in his mouth when he first mentioned it:
Reports that say that something hasn't happened are always interesting to me, because as we know, there are "known knowns"; there are things we know we know. We also know there are "known unknowns"; that is to say we know there are some things we do not know. But there are also "unknown unknowns" - the ones we don't know we don't know .
The point is this:
Do you know you know?
It is the things you don't know you don't know, which are the most dangerous. When you are learning English, be very careful. If there is just a tiny doubt that you might not be 100% sure, check it out, or get an answer from your teacher or Poligo. Too many times learning Japanese, I thought I knew but found out I was wrong. And as a teacher, I often catch my students out. It is OK to not know. You can confirm. Say something like, "Just to be sure, this is ___?" Or, maybe you could say, "This is ___, isn't it?"
The problem is sometimes you say something that is perfect English, but not what you mean. Or you think you understand someone, but really the meaning is different. A student who is wise in the ways of 英道 (Eidô) will not be caught out by the most dangerous trap of all: the unknown unknowns.
If you are interested, you can read more about Rumsfeld's words on the BBC.