The subjunctive mood in English is used to describe imagined or hypothetical situations rather than concrete facts. It is typically used in "that" clauses following verbs like suggest, advise, insist, or adjectives like desirable, crucial. For example, "I advise that you be there tonight" or "It's crucial that he sing in the performance." The subjunctive form gives these clauses a sense of something wished for, anticipated, or imagined rather than a known fact. In British English, the subjunctive is often replaced with "should" in these types of clauses.

1. A04

2.  The subjunctive in English is a verb form that's used to indicate a wish or an
imaginary situation.
 Unlike most verb tenses, it doesn't describe any particular time, and is mainly
used to describe things which aren't solid objective facts. As a result, it's often
known as the subjunctive mood, rather than tense
For example, use the subjunctive in English when talking about events that
someone:
Wants to happen.
Anticipates will happen.
Imagines happening.

4. The subjunctive mood is typically used in 'that' clauses that follow these structures:
 suggesting verb or noun + that
 advise, ask, command, demand, desire, insist, prefer, recommend, request
 advising/anxious adjective +that
 advisable, best, crucial, desirable, essential, important, necessary, vital
 anxious, determined, eager
Here's how these would look in a sentence:
suggesting verb or noun + that
 I advise that you be there tonight.
 Her boss insists that she work this afternoon.
advising/anxious adjective + that
 It's crucial that he sing in the performance.
 They are anxious that you come by soon.
These sentences all include subjunctive clauses. Everything including and following 'that' is a
subjunctive clause.

5. We can also include the word 'not' to give the
subjunctive clause a negative meaning.
• I advise that you not be there tonight.
In British English, using 'should' in place of this
subjunctive form is much more common. For example,
you might instead say:
• It's crucial that he should sing in the performance.
• Her boss insists that she should work this afternoon.