I kept the engine running because it might be necessary to leave fast.

我没有熄火，因为可能需要快速离开。

Make the necessary arrangements.

做一些必要的安排。

We will do whatever is necessary to stop them.

我们将采取一切必要行动阻止他们。

the skills necessary for writing.

写作所必需的技能。

Scientific work is differentiated from art by its necessary connection with the idea of progress.

科学工作与艺术的不同之处在于它与人类进步有着必然联系。