The subjunctive is a special kind of present tense, using an infinitive that has no –s in the third person singular. It is often used when talking about something that somebody must do.
I insist (that) your friend leave this house at once.
The subjunctive is a formal construction. It is more commonly used in American English than in British English, and more often in the written form than in the spoken form. It was used much more frequently in old English, but many of these forms have now disappeared in modern English.
That-clause
It is often used with a that-clause, especially in American English, to formally express the idea that something is important or essential.
I demand that he leave at once.


