The category of mood
The meaning of this category is the attitude of the speaker or writer towards the content of the sentence, whether the speaker considers the action real, unreal, desirable, necessary, etc. It is expressed in the form of the verb.
There are three moods in English - the indicative mood, the imperative mood and the subjunctive mood.
The indicative mood form shows that what is said must be regarded as a fact, as something which has occurred or is occurring at the moment of speaking or will occur in the future. It may denote actions with different time-reference and different aspective characteristics. Therefore the indicative mood has a wide variety of tense and aspect forms in the active and passive voice.
The imperative mood expresses a command or a request to perform an action addressed to somebody, but not the action itself. As it does not actually denote an action as a real act, it has no tense category; the unfulfilled action always refers to the future. Aspect distinctions and voice distinctions are not characteristic of the imperative mood, although forms such as, be writing, be warned sometimes occur.
The imperative mood form coincides with the plain stem of the verb, for example: Come here! Sit down. The negative form is built by means of the auxiliary do + the negative particle not (the contracted form is dont). This form is always addressed to the second person.
In commands and requests addressed to a first or third person (or persons) the analytical form let + infinitive without the particle to is used. The verb let functions as an auxiliary, and it partly loses its lexical meaning. The person addressed is expressed by the personal pronoun in the objective case.
Let us go together.
In negative sentences the analytical forms take the particle not without an auxiliary.
Let us not argue on the matter
The analytical forms differ in meaning from the synthetic forms, because their meaning is closely connected with the meaning of the pronoun included in the form. Thus let us do smth denotes an invitation to a joint action, not an order or a request. Let him do it retains to some extent the meaning of permission. In the form let me (let me do it) the first person singular does not convey any imperative meaning and should not therefore be regarded as the imperative. It conveys the meaning of I am eager to do it, allow me to do it.
The imperative mood form cant be used in questions.
The subjunctive mood is the category of the verb which is used to express non-facts: unreal or hypothetical actions or states. A hypothetical action or state may be viewed upon as desired, necessary, possible, supposed, imaginary, or contradicting reality.
Different forms of the verb are employed for this purpose.
The synthetic forms
In Old English the subjunctive mood was expressed by a special system of forms with a special set of inflections, different from those of the indicative. In the course of time, however, most of the inflections were lost, and the difference between the forms of the subjunctive and those of the indicative has almost disappeared. In Modern English there remain only two synthetic forms of the old regular system of the subjunctive, which differ from the forms of the indicative. Although their meaning and use have changed considerably, they are often called by their old names: the present subjunctive and the past subjunctive.
The present subjunctive coincides with the plain verb stem (be, go, see) for all persons in both the singular and the plural. It denotes a hypothetical action referring to the present or future. Of these surviving forms only be is always distinct from the indicative forms and is therefore rather current.