I kept the engine running because it might be necessary to leave fast. [+ to-infinitive]

我没让发动机熄火，因为也许需要迅速离开。

We will do whatever is necessary to stop them.

我们要采取一切必要措施去阻止他们。

Is that really necessary?.

那真有必要吗？

Make the necessary arrangements.

做一下必要的安排。

Wastage was no doubt a necessary consequence of war.

巨大的损耗无疑是战争的必然结果。

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

科学工作与艺术的区别在于它与人类进步有着必然联系。

...a small parcel of necessaries tied up in a handkerchief and carried on a stick.

用手绢包好挑在棍子上的一小包生活必需品

If necessary, the airship can stay up there for days to keep out of danger.

必要时，飞艇能在那里停留数天以躲避危险。

The army needs men who are willing to fight, when necessary.

军队需要在必要时愿意投入战斗的军人。

All the rigging had been examined, and renewed where necessary.

所有索具都进行了检查，并把该更换的部件全部更换一新。