You can think of the mood as the tone or manner in which something is expressed. English has three moods: the indicative, imperative, and subjunctive.
The indicative is the one we use most often. It includes statements of fact and questions:
The time passes so quickly.
We will be doing a fundraiser.
Aren’t you glad I didn’t say “banana”?
We want justice.
By that time I will have been waiting for five hours.
Indicative statements can use any of the twelve verb tenses described above.
The imperative mood covers commands, requests, and instructions. The subject of an imperative verb is implied (it’s usually you):
Do the right thing and buy me a coffee. (you do and buy)
Please join me in applauding a fine performance. (you join)
Follow these simple instructions to put together your new table. (you follow)
The subjunctive is the mood that gives writers the most trouble, in part because it is little used. These days we tend to use modal helping verbs (like should, would, etc.) to create the same effect.
The subjunctive expresses conditions that are contrary to fact, hypothetical situations that have not happened or are not likely to happen.
Here’s how the subjunctive is formed:
Now that you know how to form the subjunctive, let’s look at some situations in which the subjunctive should be used.
If I were rich, I would live in a hotel. (indicative: I was; I will live)
Wishes and desires:
I wish I were older.
Clauses that start with that and express commands, advice, wishes, and so on:
I suggest that he leave at once. (indicative: he leaves)
If you’re not sure if a statement is subjunctive, try inserting a modal helping verb that makes the clause subjunctive (that he should leave). If that works then the subjunctive mood is probably appropriate.
The subjunctive is disappearing from the English language, which is why you may often find it more natural to use the indicative form. Here’s a good example:
Subjunctive: If I be honest
Indicative: If I am honest
Most of us would pick the indicative, and not feel bad about it.
The one place where the subjunctive will likely survive for quite some time yet is in common expressions:
So be it
Be that as it may
If I were you
God help us / So help me God
Long live the King
Finally, even though the subjunctive is on the way out in English, it is frequently used in other languages.