First, understand this: The word mood has nothing to do with frame of mind, as in happy or sad. It actually refers to mode, which is the attribute of a verb suggesting the speaker's attitude toward the action expressed.
The mood of verbs shows how the speaker regards the utterance. The speaker might regard the utterance as a statement: that's the indicative mood. The speaker might ask a question: that's the interrogative mood. The speaker might issue a command: that's the imperative mood. Or the speaker might state a possibility, hope, wish, or hypothetical: that's the subjunctive mood.
Forming the subjunctive mood probably causes the most trouble. We urge you to read a discussion in Grammar.com’s section on Common Grammatical Mistakes. Click here for the beginning of that discussion. You may also download the eBook Common Grammatical Mistakes.