ㄴ, 은 (관형사형 전성 어미)

This shows up in two forms:

1. Something that already happened, and is no longer happening. (Past Tense in our language.)
2. Something that has already occurred to something. (Present Tense for a descriptive word.)

Take the case of 예쁜 꽃. Translated simply as "pretty flower", what this really says to a Korean is that at some point in the past, the flower may not have been pretty, but since that time, it has become pretty.

This gets to the heart of why Koreans think of their adjectives as verbs: These are things that happened to the objects, a sort of passive event. It only makes sense, then, that in describing something the way it is now, you would use the past tense of the verb that caused it to be so.