Basically, the idea is that when the sentence has the subject giving a command, the verb tense should be "to be" and not a variation of this. This article talks about some exceptions, but the majority of the examples you see on the GMAT should follow the general "to be" rule.
Think of it this way: Command subjunctive applies when a subject is commanding that something be a certain way. Therefore, "to be" is your go-to verb. If the meaning of the sentence doesn't work with "to be," then it's okay to use a different verb form.
Mom expects that the room be cleaned by the time she comes home.
It's essential to pick the children up by 10.
Does this help?