1. worth： be worth + n.　 当名词为金钱时，表示"…… 值得……"

　　be worth doing sth.　"……某事值得被做"

　　The question is not worth discussing again and again.

　　2. worthy：be worthy of +n.　当名词为抽象名词时表示"……值得……"

　　be worthy to be done "某事值得被做"

　　The question is not worthy to be discussed againand again.

　　3. worth-while： be worth-while to do sth　 "值得做某事"

　　worth while： It is worth while doing sth

　　It is worth while sb to do sth.

　　典型例题

　　It is not ____ to discuss the question again and again.

　　A.worth　B.worthy　C.worth-while　D.worth while

　　答案C. 由worth的用法可知，此句只适合词组be worth-while to do sth.。因此选C。