The subjunctive is a special verb form used for things that are not being stated as facts — wishes, doubts, imaginary situations, and demands about what someone should do. English still has it, but it hides, because in most sentences the subjunctive form looks exactly like a form you already know. You meet it in two main places. First, in unreal 'if' sentences and wishes, where 'was' becomes 'were': 'If I were you, I'd take the job', 'I wish it were Friday'. Second, after verbs and adjectives of asking, ordering, and advising, where the verb loses its ending: 'The doctor insists that he rest', 'It's essential that she be there by nine' — not 'rests', not 'is'. The word is both a noun ('the verb is in the subjunctive') and an adjective ('the subjunctive mood'). Now the honest part your grammar book may not say: in real speech many people say 'If I was you', and in Britain 'The doctor insists that he should rest' is more usual than the bare 'rest'. Americans keep the bare subjunctive more strongly. So the subjunctive is alive in writing and in fixed phrases ('God save the Queen', 'be that as it may', 'come what may', 'if need be'), while everyday speech often works around it. In Spanish, French, and other languages it is used far more, and it is much more visible there.