關於英文necessity用法問題。?

necessity是將形容詞necessary字尾-ary換上名詞字尾-ity所形成的「抽象名詞」，而抽象名詞是無複數形，不加a/an等，表示總稱時，不加the。用以指特定的事物時，則需加上the。
（見柯旗化《新英文法》pp.10-12）
但有通則，就常會有例外。
字義就是一個會影響到抽象名詞成為可數名詞的因素。

necessity = [U] the need for something時是「不可數名詞」
With a personal fortune of six million pounds, she certainly doesn't work out of necessity.
The report stresses the necessity of eating plenty of fresh fruit and vegetables.

necessity = [C] something that you need, especially in order to live
He regarded music as one of life's necessities. 
這時屬於「可數名詞」，有複數。

見p.829 Cambridge Advanced Learner's Dictionary

A book is a necessity when traveling. 
necessity = an indispensable thing

見p.1171 New Oxford American Dictionary (3rd edition)

necessity :   (from Oxford dictionary)
[countable, usually singular] a situation that must happen and that cannot be avoided (一種必須發生而無法避免的情況)

Night follows day as a necessity.
necessity在這句是屬可數名詞[C]，所以前面一定要加上不定冠詞"a".