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

I was then think photographer is a small but indispensable role, wish with all my heart to become a photographer.

我当时便觉得摄影记者是一个微小却又不可缺少的角色，一心希望成为一个摄影记者。

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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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