lack 指完全短缺或数量不足。
need 语气较重，指需要必不可少的东西，强调急需。
want 侧重缺少某种必需之物，或个人渴望得到的东西。
require 使用广泛，语气较轻。强调急需时可与need换用，但有时暗示所需的人或物是完成某一任务必不可少的。
参考例句
1. He relieved us when we needed. 在我们最困难的时候，他救济了我们一家。
2. A notebook is all that I need. 一个笔记本就是我所需要的。
3. Do you think I really need it? 你认为我的确需要做体检吗?
英英解释
n.
1. a condition requiring relief
2. anything that is necessary but lacking
3. the psychological feature that arouses an organism to action toward a desired goal; the reason for t
4. a state of extreme poverty or destitution v.
1. require as useful, just, or proper
2. have need of
3. have or feel a need for
