Strange to say, but verbs, like people, have moods. Let’s look at these three short sentences and see how they differ in form and purpose: You’re always on time, Always be on time, and I wish you’d been on time.
Let’s begin by understanding what is meant by the word mood. As a grammatical term, mood is derived from the Latin word modus, meaning manner, way, or measure (meaning a state of mind or feeling, the word mood has a different derivation). When we speak of the mood of a verb, we mean to point to the way in which it is stating its thought. To say You’re always on time is to state a thought as a fact, something presumed by the speaker to be true, though evidence to the contrary could perhaps be offered to correct the assertion. When we intend to state a fact, our verb is constructed in what is called the indicative mood: we state the subject (you) and we connect it with its verb (are) in whatever tense might be appropriate to the circumstances. Mood and tense are different properties of a verb, so the mood can stay the same while the tense may differ (You were always on time, You will always be on time).
Not every sentence we speak or write, though, intends to state a fact. Sometimes we wish to tell someone what to do, and when that’s the case, we construct the sentence differently. If I have managed finally to find a job and a friend warns me by saying, Always be on time, he in his good heart is not pointing to an existing fact, but is very much concerned that my always being on time will become a real and dependable fact. To command or direct someone to act or exist in a certain way, we must change the verbal mood from the factual indicative to what is called the imperative. We do this most often by using the infinitive form of the verb and simply not stating the subject. If we analyzed the sentence Always be on time, we would have to place the vertical bar separating subject from predicate at the very beginning of the sentence, because there is no subject stated: | always be on time. This signals a command.
But facts, present or commanded, are not the only things we can talk about. We can also express our feelings about something that in fact wasn’t the case but we feel should have been. To say I wish you’d been on time is to state a fact (my wishing) about something that wasn’t a fact (your being on time). Complex sentences like this have at least two clauses, the first in the indicative mood (because my wishing is certainly a fact) and the second in what is called the subjunctive mood. This is the mood we use to positively state what was not the case in relation to my own hopes and dreams, that is to say, in relation to my interior life. There are a number of ways to construct the subjunctive mood, but the one at work in our sentence here involves using tense in an unexpected way. The verb in the clause you had been on time is in the past perfect tense, but the speaker is simply referring to a time before the time he is expressing his present wish. That would be a mistake in an indicative sentence (where we would be required to use the simple past), but that irregular use of tense is just how the subjunctive mood is expressed at times—the misalignment of time signals the fact that we are in the internal subjective world of the speaker, not the external, factual world we all rationally share.
There are, then, three moods to a verb in English: indicative, imperative, and subjunctive. Mood is one of the five properties of a verb (person, number, tense, and voice are the others), and since a property is an essential feature of something, it is essential we understand how to construct the mood of a verb to ensure that what we are saying is said in the manner we intend—whatever our mood.