Even grammar can be "moody," but grammatical moods express the attitude of what a person is writing or saying. The three grammatical moods commonly used in English are the indicative, imperative, and subjunctive moods.

The indicative (or realis) mood is used to make a statement of fact:
 

You cannot outlast us and we will defeat you.

Caption 19, Barack Obama's Inauguration Day - Obama's Speech

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You will learn the true nature of the society we live in.

Caption 41, George Orwell's Nineteen Eighty-Four - BBC TV Movie

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The imperative mood is for commands or requests:
 

Step away from your vehicle and put your hands on your head.

Captions 10-11, Movie Trailers - Men In Black

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All emergency service cars, please come to Vesey and West [Streets]!

Caption 4, World Trade Center - Story on the 2006 Film

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The subjunctive mood is used to express a a wish, desire, or something that has not yet happened. 
 

I'd like to have something interesting to do and I'd like to have nothing to do.

Caption 54, Leonard Nimoy - Talking about Mr. Spock

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I would like to explain how we talk about the time in English.

Caption 3, Lydia Explains - The Clock

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Further Learning
Read more about grammatical moods and find examples on Yabla English to see them used in a real-world context.