What Is Grammatical Mood?
The term “mood,” when used in a grammatical sense, refers to verb forms that illustrate the way the speaker is treating an action. In Proto-Indo-European, there were four moods:
Indicative mood — fact
Subjunctive mood — idea
Optative mood — wish
Imperative mood — command
The indicative mood treats an action as a fact. The word “treats” is important because whether an action is indeed a fact or not is beside the point. When a speaker uses the indicative mood, he or she is only treating the action as a fact.
Rī mā nī hostēs vī cē runt.
The two verb tenses you've learned so far — the imperfect and the perfect — have both been in the indicative mood. In this chapter you will learn two tenses — the imperfect and the pluperfect — but in the subjunctive mood.
With the subjunctive mood, the speaker is treating an action as an idea.
That is to say, as a sort of hypothetical situation.
Si Rī mā nī hostēs vincerent, …
Once again, the factual or potential truth of what is being said doesn't really matter.
Just like the old Proto-Indo-European ablative, instrumental, and locative cases merged into one case (the ablative) in Latin, the old optative mood combined with the subjunctive in Latin. This gives Latin one mood, the subjunctive, that does the work of two.
Utinam Rī mā nī hostēs vincerent. (I wish that the Romans conquered the enemy.) (If only the Romans conquered the enemy.) (Would that the Romans conquered the enemy.)
Since wishes are ideas and not facts, it's easy to see how the subjunctive and optative moods were able to merge.
The fourth mood, the imperative, simply expresses a command.
Rī mā nī,vincite hostē s! (Romans, conquer the enemy!)
The imperative mood is covered in detail in Chapter 18.
The subjunctive is used occasionally in English. For example, in the sentence “She insists that you be ready by noon,” why is it “you be” rather than “you are”? “She insists” is a fact, “you be” is a wish. Some other examples are “I wish Katie were my physician” and “come hell or high water.”
There is a fifth category that isn't exactly a mood, but it's worth mentioning here. This group contains forms that are called “infinitives.” Infinitives are verb forms that don't treat actions as anything at all. They don't even have a person or number! In English, infinitives are usually preceded by the word “to,” as in “to walk,” “to cry,” “to sweep,” and so on. Infinitives are covered in detail in Chapter 17.