根据短文内容，用方框中所给的单词的适当形式填空，每词限用一次。

aim   find   fortunate   either   realize

  Most parents, I suppose, have had the experience of reading a bedtime story to their children. And they must _______ how difficult it is to write a good children’s book. Either the author has _______ too difficult, so that children can’t follow what is in his story, or the story seems to be talking to the readers.
The best children’s books are ________very difficult nor very simple, and satisfy the child who hears the story and the adult who reads it. ________, there are in fact few books like this, so the problem of ________ the right bedtime story is not easy to solve.