Grammatical mood
Many languages have the concept of grammatical mood, which describes the talker's relation, or intention to the reality. Many times this relation is reflected by the inflection of the verb. Modern English does not have all of these moods, and has a very simplified verb inflection, thus it's not straightforward to explain them in this language, and different languages might even define them a bit differently.
Possible moods include:
Indicative mood
Used to express facts and opinions.
Imperative mood
Used to give commands, direct requests, prohibitions.
Subjunctive mood
Used in dependent clauses, when talking about hypothetical, unlikely events. It exists in English but is often not used.
Injunctive mood
?
Optative mood
Used to express potential events, or wishes.