Since adults know how to place the key in the key slot, which is the ultimate purpose of approaching the slot, and since the child is neither old enough nor clever enough to realize the desired action on his own, what possible gain is achieved by having him struggle? He may well get frustrated and angry — certainly not a desirable outcome.

请问专家老师们，desired 和desirable 作形容词，用法有何区别？作者如何甄选的？谢谢。