While means to figure out the content of something or to be
clear or to be understandable, ftl€> means to find out something or to become acquainted with something or someone.
Do you understand Spanish?
Do you know Spanish?
ft] 5 .with its basic meaning of to find out or to become acquainted, is translated as know when itappears in its-Tl^-S form (lit., /s in the state of having found out or having become acquainted). This is why you use Sl^Tl^ for knowing people and being familiar with places and ideas.
Do you know Mr. Kawamura's address?
bhgsh
No, I don't. But I'll know (lit, it will be clear) if I check it,
Both £>fr U and ft] ij £-r±,A; can be used for / don't know,
but the nuance is quite different. ft] U jfe-t+A/ can imply I don't know and have no reason to, so it can sound a bit rude if the question is one that you could be expected to know the answer to. That is why, when you ask a store clerk a question about the merchandise, he or she answers with k>tf> U Sit/ufa instead of ft] U %.HAj. (The makes it clear that the meaning is / don't know, not / don't understand you.)
In fact, saying ftlb&lN directly to another person's face is a way of saying / disapprove of what you are doing and want nothing to do with you. Furthermore, since ft]-5 refers to finding out something that you didn't have any reason to know, it sounds odd to use ft] D Sit ho in reference to something concerning yourself, such as your own future plans or your own wants
However, ft] U StirAy is a perfectly acceptable response when you don't know a person, are unfamiliar with a place or idea, or have no expertise in a certain subject area