We prefer that the plan should be fully discussed before being put into execution.这是完整的句子 其中should可以省略 因为prefer 所以要用虚拟语气
这里省略了一个would,plan would be fully discussed,这里would可以省略
计划被讨论,所以要用be
虚拟句所以discuss用过去时
(should) be fully discussed ,should 可以省略,一般用should,表示虚拟语气