Mood is distinct from grammatical tense or grammatical aspect, although the same word patterns are used for expressing more than one of these meanings at the same time in many languages, including English and most other modern Indo-European languages. (See tense–aspect–mood for a discussion of this.)
Some examples of moods are indicative, interrogative, imperative, subjunctive, injunctive, optative, and potential. These are all finite forms of the verb. Infinitives, gerunds, and participles, which are non-finite forms of the verb, are not considered to be examples of moods.
Some Uralic Samoyedic languages have more than ten moods; Nenets has as many as sixteen. The original Indo-European inventory of moods consisted of indicative, subjunctive, optative, and imperative. Not every Indo-European language has all of these moods, but the most conservative ones such as Avestan, Ancient Greek, and Sanskrit have them all. English has indicative, imperative, and subjunctive moods; other moods, such as the conditional, do not appear as morphologically distinct forms.
Not all of the moods listed below are clearly conceptually distinct. Individual terminology varies from language to language, and the coverage of (e.g.) the "conditional" mood in one language may largely overlap with that of the "hypothetical" or "potential" mood in another. Even when two different moods exist in the same language, their respective usages may blur, or may be defined by syntactic rather than semantic criteria. For example, the subjunctive and optative moods in Ancient Greek alternate syntactically in many subordinate clauses, depending on the tense of the main verb. The usage of the indicative, subjunctive, and jussive moods in Classical Arabic is almost completely controlled by syntactic context. The only possible alternation in the same context is between indicative and jussive following the negative particle lā.
Realis moods are a category of grammatical moods that indicate that something is actually the case or actually not the case. The most common realis mood is the indicative mood. Some languages have a distinct generic mood for expressing general truths. For other realis moods, see the main Realis mood article.
The indicative mood, or evidential mood, is used for factual statements and positive beliefs. It is the mood of reality. The indicative mood is the most commonly used mood and is found in all languages. Example: "Paul is eating an apple" or "John eats apples". All intentions that a particular language does not categorize as another mood are classified as indicative.
Irrealis moods are the set of grammatical moods that indicate that something is not actually the case or a certain situation or action is not known to have happened. They are any verb or sentence mood that are not realis moods. They may be part of expressions of necessity, possibility, requirement, wish or desire, fear, or as part of counterfactual reasonings, etc.