indispensable

Reliable internet access has become indispensable for modern business operations.

(Chinese) 可靠的互联网连接已成为现代商业运营中不可或缺的一部分。

A good dictionary is an indispensable tool for any serious language learner.

(Chinese) 对于任何认真的语言学习者来说，一本好字典都是不可替代的工具。