While means to figure out the content of something or to be
clear or to be understandable, £P -5 mea ns to find out something or to become acquainted with something or someone.
Do you understand Spanish?
Do you know Spanish?
£P    with its basic meaning of to find out or to become acquainted, is translated as/raw when itappears in its-Tt^-S form (lit, is in the state of having found out or having become acquainted). This is why you use fcp-pTl^ for knowing people and being familiar with places and ideas.
-ji7A72/v<Di£ fefctUSttflo Do you know Mr. Kawamura's address?
No, I don't. But I'll know (//t, it will be clear) if I check it.
Both fofr IJ %-\kfv and ft] IJ £i+Av can be used for / don't know, but the nuance is quite different. $P CJ j£-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 t)2MJ Stir/ufa instead of 3EP y StirA;. (The makes it clear that the meaning is / don't know, not / don't understand you.)
In fact, saying ftlb&l^ 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 refers to finding out something that you didn't have any reason to know, it sounds odd to use £P lJ j£"t±A> in reference to something concerning yourself, such as your own future plans or your own wants.
However, $P lJ 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.