A keycard is necessary for entry to the room.
门卡是进这间房子所必须的。


Food and water are necessary for life.
食物和水是生命所需。

As his story drew to a close it became more and more necessary for me to interrogate him about himself.

他的叙述要结束时，我越发需要询问他自己的事情。

Do you think it's necessary to know different customs in different countries?

你觉得有必要了解不同国家的不同风俗习惯吗？

※To travel through the world it is necessary to have the mouth of hog, the legs of a stag, the eyes of a falcon, the ears of an ass, shoulders of a camel, and the face of an ape, and overplus, a satchel full of money and patience. 周游世界须有猪嘴巴，鹿的腿，鹰的双
※Knowledge is a comfortable and necessary retreat and shelter for us in an advanced age; and if we do not plant it when young, it will give us no shade when we grow old. 学识可使老年时舒适地退隐和有所寄托；但如果年青时不使它扎下根，老年就得不到它的庇护。
※A certain amount of care or pain or trouble is necessary for every man at all times .A ship without a ballast is unstable and will not go straight.——Arthur Schopenhauer.Geman philosopher一定的忧愁、痛苦或烦恼，对每个人都是时时必需的。一艘船如果没有压舱物，便不会稳定，不能朝着目的地一直前进。德国哲学家  叔本华  A
※Make yourself necessary to someone. 使你有益于人。