Verbs in English (and many other languages) have a characteristic called “mood.” When you make a statement or ask a question, the mood of the verb is indicative. When you directly tell someone to do something, the mood of the verb is imperative.
With a bunch of grammar “stipulations,” when you are trying to persuade someone to do something, the verb in the dependent clause is subjunctive.
…He GOES with her. (indicative)
…GO with her. (imperative)
…I insisted that he GO with her. (subjunctive)
And the subjunctive drops the “s” from the verb for third person singular.
…It is imperative that he LEAVE immediately.
…He demands that she ARRIVE on time.