The subjunctive (also known as the conjunctive in some languages) is a grammatical mood, a feature of an utterance that indicates the speaker's attitude toward it. Subjunctive forms of verbs are typically used to express various states of unreality, such as wish, emotion, possibility, judgment, opinion, obligation, or action, that has not yet occurred. The precise situations in which they are used vary from language to language. The subjunctive is one of the irrealis moods, which refer to what is not necessarily real. It is often contrasted with the indicative, a realis mood which principally indicates that something is a statement of fact.

Subjunctives occur most often, although not exclusively, in subordinate clauses, particularly that-clauses. Examples of the subjunctive in English are found in the sentences "I suggest that you be careful" and "It is important that she stay by your side."

The Proto-Indo-European language, the reconstructed common ancestor of the Indo-European languages, had two closely related moods: the subjunctive and the optative. Many of its daughter languages combined or merged these moods.

In Indo-European, the subjunctive was formed by using the full ablaut grade of the root of the verb and appending the thematic vowel *-e- or *-o- to the root stem, with the full, primary set of personal inflections. The subjunctive was the Indo-European irrealis, used for hypothetical or counterfactual situations.

The optative mood was formed with a suffix *-ieh₁ or *-ih₁ (with a laryngeal). The optative used the clitic set^{[clarification needed]} of secondary personal inflections. The optative was used to express wishes or hopes.

Among the Indo-European languages, only Albanian, Avestan, Ancient Greek, and Sanskrit kept the subjunctive and the optative fully separate and parallel. However, in Sanskrit, use of the subjunctive is found only in the Vedic language of the earliest times, and the optative and imperative are comparatively less commonly used. In the later language (from c. 500 BC), the subjunctive fell out of use, with the optative or imperative being used instead, or merged with the optative as in Latin. However, the first-person forms of the subjunctive continue to be used, as they are transferred to the imperative, which formerly, like Greek, had no first person forms.

In the Germanic languages, subjunctives are also usually formed from old optatives (a mood that indicates a wish or hope), with the present subjunctive marked with *-ai- and the past with *-ī-. In German, these forms have been reduced to a schwa, spelled -e. The past tense, however, often displays i-umlaut. In Old Norse, both suffixes evolved into -i-, but i-umlaut occurs in the past subjunctive, which distinguishes them.

In Modern English, the subjunctive is realised as a finite but tenseless clause where the main verb occurs in the bare form. Since the bare form is also used in a variety of other constructions, the English subjunctive is reflected by a clause type rather than a distinct inflection.