necessary n．必需品 adj. 必要的，必需的，必然的
例句：The necessary outcome of a war is a fall in production.
战争带来的必然结果就是生产力下降。

indispensable n．不可缺少之物 adj. 不可缺少的，绝对必要的
例句：The computer is now an indispensable tool in many businesses as well as in many schools.
目前计算机是很多公司和学校不可或缺的用具。

imperative n．命令，需要 adj．命令的，强制的
例句：Job creation has become an imperative for the government, especially under the strike of national financial crisis.
创造就业机会是政府必须做的事，特别是在全国性金融危机的冲击之下。

obligatory adj．义不容辞的，必须的
例句：It is obligatory for client to supply the personal data as required by the company.
客户有责任向本公司提供所需的个人资料。

oblige vt．迫使，责成，施恩惠
例句：Joseph felt obliged to leave after such an unpleasant quarrel.
发生了这样不愉快的争吵之后，约瑟夫觉得有必要离开。

obligation n．义务，职责，债务
例句：Damaging the goods puts you under an obligation to buy them.
你损坏了这些商品，你就得都买下来。

necessarily adv. 必要地
例句：Try as Angela may, she is not necessarily able to talk him out of smoking.
虽然安吉拉会尝试，却未必能说服他戒烟。

necessitate v．成为必要
例句：Your proposal will necessitate borrowing more money.
依你的建议就必须增加借款。

necessity n．必要性，需要，必需品
例句：The doctor asked us not to call him during the night except ln case of necessity,
医生吩咐我们，除非必须，否则不要在夜里叫他。