The word mood is derived from the Latin word ‘modus’ which means mode or manner. Hence the mood of a verb shows the mode or manner in which the action expressed by the verb takes place.

There are three main moods in which an action can take place and they are as follows:

The indicative mood
The imperative mood
The subjunctive mood

The indicative mood

The indicative mood is used to make a simple statement or to ask a question. It is also used to make a possible supposition.

I wake up early in the morning. (Statement)
Sun rises in the east. (Statement)
Barking dogs seldom bite. (Statement)
Milk is sweet. (Statement)
What is your name? (Question)
Is that your husband? (Question)
If you ask him, he will help you. (Possible supposition)

Imperative mood

The imperative mood is used to express a command, request or advice.

Go at once. (Command)
Sit down.
Excuse me. (Request)
Keep quiet. (Order)
Don’t be silly. (Advice)
Word hard. (Advice)

Notes:

The imperative mood is used in the second person. Note that the subject you is usually omitted in the imperative mood.

(You) get out!
(You) shut up!

Note that if the command is addressed to the person speaking (First person), it will begin with let followed by me or us.

Let me go now.
Let me have a look at your essay.
Let us start now.

Third person commands will begin with let preceded by the third person subject.

Let somebody help me.

The subjunctive mood

The subjunctive mood is used to express an impossible supposition or an ardent desire or wish.

If I were you I would not act in this manner. (An impossible supposition)

The subjunctive mood may also express a wish.

Long Live Revolution!
May God bless you!
I wish I were a millionaire.

The subjunctive mood may appear in the subordinate clause of a conditional sentence:

If wishes were horses, beggars would ride them.
If he arrived in time, he would be able to meet the President.

Notes

The subjunctive mood is now rarely used. It usually only follows a verb like suggest, insist, demand or propose. Note that when a verb is used in the subjunctive mood, it does not have –s in the third person singular.

I suggest that she refuse the offer. (NOT I suggest that she refuses the offer.)
They demanded that she reveal the secrets. (NOT They demanded that she revealed the secrets.)

To avoid this ambiguity the modal auxiliary verb should is sometimes inserted into the sentence.

I suggest that she should refuse the offer.
They demanded that she should reveal the secrets.

When a verb is in the subjunctive mood be is used in the present tense and were in the past tense.
I insist that I be freed. OR I insist that I should be freed. (NOT … I am freed.)
If I were you I would accept this offer. (NOT If I was …)