The former is principal and basic but not on ly, the latter is the second but indispensable.

前者是主要的或基本的，但不是唯一的，后者是第二位的，却是不可或缺的。

youdao

In the modern tourism industry, each kind of advertisement is not dispensable, but indispensable, and the promotion function of the tourism advertisements will be more and more obvious.

在现代旅游业中，各种广告已经不是可有可无，而是非有不可，旅游广告的促销作用将得到越来越大的显现。

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But policies to help workers adapt will be indispensable.

但帮助工人去适应的政策将是必不可少的。

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The Supreme Court considered the swearing-in an important formality, but not indispensable for the start of the new presidential term, she said.

BBC: Chavez swearing-in delay legal, rules Venezuela court

"These people tend to be highly responsible individuals who fundamentally believe that they're not only important to others but also indispensable, " Mr. Davis says.

WSJ: When Job Fatigue Hits the CEO

That has opened the way to restoring the earnings link to the basic state pension, another no-go area for the chancellor, but an indispensable reform if means-testing is to be arrested and saving encouraged.

ECONOMIST: Reforming pensions