In their variety of forms, conditional sentences may be used to describe reality and unreality as well as to express certainty, probability and impossibility. Therefore the occurrence of different moods in these structures is not surprising.

Mood is generally defined as a grammatical category expressing the relation of the action to reality as stated by the speaker. The function of marked moods, in opposition to the indicative, is to represent the speaker's utterance not as a real fact but as a wish, supposition, doubt or condition which is hypothetical or contrary to the fact. Employing one of the marked moods the speaker communicates to the hearer what he/she considers improbable, probable or desirable. (Hence the difference between the marked moods and the imperative: Stop talking! means an order directly addressed and to be fulfilled, whereas I wish you stopped talking is merely information of the speaker's hopes or desires.)

The distinction between the real and unreal by means of various verb forms is still one of the controversial problems of English grammar. The main difficulty is due to the coexistence of both synthetic and analytical forms of the verb with the same grammatical meaning and to the fact that there are verbal forms homonymous with the Simple Past and Past Perfect of the indicative mood which are used to express unreality. The other problem consists in distinguishing the analytical forms of the subjunctive with the auxiliaries should, would, may/might, which lack any lexical meaning, from the homonymous verb groups in which the lexical meaning of these verbs is preserved.