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.
戰爭帶來的必然結果就是生產力下降。

indispensable n．不可缺少之物 adj. 不可缺少的，絕對必要的
例句：The computer is now an indispensable tool in many businesses as well as in many schools.
目前計算機是很多公司和學校不可或缺的用具。

imperative n．命令，需要 adj．命令的，強制的
例句：Job creation has become an imperative for the government, especially under the strike of national financial crisis.
創造就業機會是政府必須做的事，特別是在全國性金融危機的沖擊之下。

obligatory adj．義不容辭的，必須的
例句：It is obligatory for client to supply the personal data as required by the company.
客戶有責任向本公司提供所需的個人資料。

oblige vt．迫使，責成，施恩惠
例句：Joseph felt obliged to leave after such an unpleasant quarrel.
發生了這樣不愉快的爭吵之后，約瑟夫覺得有必要離開。

obligation n．義務，職責，債務
例句：Damaging the goods puts you under an obligation to buy them.
你損壞了這些商品，你就得都買下來。

necessarily adv. 必要地
例句：Try as Angela may, she is not necessarily able to talk him out of smoking.
雖然安吉拉會嘗試，卻未必能說服他戒煙。

necessitate v．成為必要
例句：Your proposal will necessitate borrowing more money.
依你的建議就必須增加借款。

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,
醫生吩咐我們，除非必須，否則不要在夜里叫他。