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.
Most sentences appear in the indicative mood, regardless of tense. The following show the indicative mood:
I write the book. (present tense)
I wrote the book. (past tense)
I will write the book. (future tense)
I will be writing the book. (future-progressive tense)