essential/essentially

essential /iˈsenʃəl/ I. adj. 重要的，必不可少的；基本的，本质的：her most essential quality 她最根本的品质 ［考句］It is essential that these application forms be sent back as early as possible. 这些申请表必须尽早寄回。（［注］在it be essential that…中，虚拟语气形式为should+动词原形，should可省略。）‖II. n. ［c］ 必需的东西；本质部分(常用pl.)：Petroleum is an essential of modern industry. 石油是现代工业所必需的。The book deals with the essentials of English grammar. 此书涉及的是英语语法最本质的东西。

essentially adv. 基本地，本质地：［考句］These goods are essentially for export, though a few of them may be sold on the home market.这些商品基本上是出口的，尽管其中一部分也在国内市场销售。