[译] 士兵必须服从命令。
[误] It is essential that a soldier must obey orders.

[正] It is essential that a soldier should obey orders.
[注] 在“It is essential that + 从句”的句子结构中，由 that 引导的从句中的谓语动词必须用虚拟语气，即“should + 动词原形”，其中 should 可以省略。