English reserves the subjunctive mood for phrases expressing desires, conditions, imaginations, or counterfactual ideas. When in present tense, verbs should take the base (infinitive) form of to construct a subjunctive clause: I suggest that he write, I demand that you be present, It is necessary that each employee wash his or her hands. In past subjunctive tense, verbs take the simple past form, except be, which becomes were: I wish I were smarter, If only he took the bus. In pluperfect tense, verbs take the past-perfect mood: If he had taken the bus. Subjunctive phrasing provides a sense of uncertainty and contrary-to-reality feel that one does not achieve through indicative phrasing. Compare If he lies, I will leave (indicative) to If he lied (or should lie or were to lie), I would leave (subjunctive).
Counterfactuals (also called contrary-to-fact conditions) appear most commonly in conversation and, likely a result, are often misused. Counterfactuals form as wishes or imaginations, often preceded by if, as if, wish, or words of similar effect: If I were wealthy, I would not be working.
While subjunctives see rare use in our language, they do lend themselves to a few popular apothegms: God bless America, God save the Queen, Lord help us, and be that as it may.