English verbs have four moods: indicative, imperative, subjunctive, and infinitive.
Mood is the form of the verb that shows the mode or manner in which a thought is expressed.
1. Indicative Mood: expresses an assertion, denial, or question:
Little Rock is the capital of Arkansas.
Ostriches cannot fly.
Have you finished your homework?
2. Imperative Mood: expresses command, prohibition, entreaty, or advice:
Don’t smoke in this building.
Don’t drown that puppy!
3. Subjunctive Mood: expresses doubt or something contrary to fact.
Modern English speakers use indicative mood most of the time, resorting to a kind of “mixed subjunctive” that makes use of helping verbs:
If I should see him, I will tell him.
Americans are more likely to say:
If I see him, I will tell him.
The verb may can be used to express a wish:
May you have many more birthdays.
May you live long and prosper.
The verb were can also indicate the use of the subjunctive:
If I were you, I wouldn’t keep driving on those tires.
If he were governor, we’d be in better fiscal shape.
4. Infinitive Mood: expresses an action or state without reference to any subject. It can be the source of sentence fragments when the writer mistakenly thinks the infinitive form is a fully-functioning verb.
When we speak of the English infinitive, we usually mean the basic form of the verb with “to” in front of it: to go, to sing, to walk, to speak.
Verbs said to be in the infinitive mood can include participle forms ending in -ed and -ing.
Verbs in the infinitive mood are not being used as verbs, but as other parts of speech:
To err is human; to forgive, divine. Here, to err and to forgive are used as nouns.
He is a man to be admired. Here, to be admired is an adjective, the equivalent of admirable. It describes the noun man.
He came to see you. Here, to see you is used as an adverb to tell why he came.
_From English Grammar 101_