necessary和necessity的分別

necessary = 必需
e.g. Food is necessary for people to live. (人必需有食物才可以生存。)

necessity 必要性/必需品
e.g. 1) He felt the necessity of finding a job. (他感到必須找到工作。)
2) PC is considered a necessity by a lot of people. (許多人認為電腦是一件生活必需品。)

1.necessary=1. 必要的,必需的/必然的,無法避免的
2.necessity=需要,必要性/必需品

Fundamental difference.
necessary is an adjective
necessity is a noun

optional info.
necessitate is a verb
necessarily is an adverb