(1)必要(需)的(needed to achieve a certain result or effect)[A(to，for)]：Water is～to man. 水对人来说是必需的。This bit of knowledge is very～to a student of British history. 这点知识对学英国史的学生是非常必要的。Sleep is～to health. 睡眠是健康的必要保障。Was it～for you to go yesterday? 你昨天有必要去吗? Decision is～for a commander. 作为一名指挥官必须坚定果断。They must obtain the two-thirds majority～for ratifying the agreement. 他们必须得到三分之二的多数票才能使协议获得批准。Money is～in life. 金钱在生活中是必不可少的。Fill out the～forms. 请填好这些必要的表格。Is it～to bring our own food ? 有必要自己带饭吗? 〖同〗required，needed，essential;〖反〗unnecessary，nonessential，optional;

(2)必然的;不可避免的(following as a certain result;unavoidable)：the～consequence/end/result/truth 必然的后果/必然的结局/必然的结果/不容否定的真理;Crowded streets and shops are a～part of big-city life. 拥挤的街道和商店是大城市生活中不可避免的。There is no～connection between industrial democracy and productivity. 工业民主和生产率之间没有必然的联系。

a necessary evil 无法回避的坏事;不好但又必不可少的东西(人)：They looked upon him as a～evil. 在他们看来，他很讨厌，但又少不了他。They call the social evil a～evil. 他们称这种社会弊病是不可避免的顽疾。

→′necessarily adv 必定地;自然地;

【辨异】necessary与essential的区别见ESSENTIAL。