都表示必需，需要之意区别

1. essential 表示语气更强力，至关重要，表示没有不行的含义

• 　　It was absolutely essential to separate crops from the areas that animals used as pasture.
• 　　The apartment contained the basic essentials for bachelor life.

2. necessary 表示需要，意思和need相近，但是没有到达没有不行的地步

• 　　I kept the engine running because it might be necessary to leave fast.
• 　　Scientific work is differentiated from art by its necessary connection with the idea of progress.

引用出处：

http://www.differencebetween.net/language/words-language/difference-between-essential-and-necessary/

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