imperative

我必须立即和他谈话，这至关重要。

It is ___imperative that I speak with him at once.

路标上的 Stop 一词是祈使语气（命令式）。

The word 'Stop' on a road sign is an ___imperative.

在这场危机中，迅速做出决定是至关重要的。

In this crisis, it is ___imperative to make a swift decision.