Sentences like "It's essential that he be informed" or "If I were you, I'd leave" are examples of the subjunctive.
The subjunctive is more common in American English and formal British English than it is in spoken or informal British English. It's used to refer to situations that are not real (but are desirable) or hypothetical situations.
We typically use the subjunctive after verbs such as recommend / suggest / propose / insist / demand / ask or phrases such as it is essential / vital / necessary that...
I suggest that he examine the proposal carefully.
They demanded that he leave immediately.
I ask that you be vigilant about security.
It is vital that we accept this offer.
The subjunctive is also common in conditional sentences, such as "If I were you, I'd..." or "If he were..." etc. Again, this is far more common in American English than in British English where you might also hear "If I was you" or "If he was..."
Other examples of hypothetical sentences using the subjunctive are with "suppose", "wish", "if" and "as if":
Suppose I were your boss. How would you feel?
I wish it were easier to explain, but...
If it weren't for his quick thinking, we would have been in great danger.
He talks about his country as if it were a paradise
How to form the subjunctive
Use the base form of the verb (without "to"). This form is the same for I, you, he / she, we or they, and can be used to describe either present or past situations. The exception is the verb "to be" where the past subjuntive form is "were".
Avoiding the subjunctive
British English speakers will typically avoid the subjunctive by using a form such as "should":
I suggest that he should examine...
They demanded that he should leave...
Alternatively, a gerund is also possible with verbs such as suggest / propose / recommend / insist + on
I suggested examining...
I recommended taking...
They proposed delaying the decision.
He insisted on speaking.