It's difficult to define the exact nature of the problem.

　　很難給這個問題確切定性。

　　We need to define the task ahead very clearly.

　　我們需要明確今后的任務。

　　The powers of the police must be clearly defined.

　　警察的職權必須明確界定。

　　It is difficult to define the border between love and friendship.

　　愛情和友情之間的界線難以劃清。

　　These categories are not well defined.

　　這些類別劃分得不太明確。