Possible moods include indicative, imperative, subjunctive, injunctive, and optative.
The 'indicative mood express facts and opinions. It is the most commonly used mood and is found in all languages. Example: "Paul is reading".
The imperative mood expresses commands, direct requests, prohibitions. In many circumstances, directly using the imperative mood seems blunt or even rude, so use with care. Example: "Paul, read that book".
The subjunctive mood has several uses in dependent clauses, when discussing hypothetical or unlikely events, or for polite requests. A subjunctive mood exists in English but many native English speakers have not mastered it. Example: "If Paul were reading the book, he would be occupied". Paul is not in fact reading.
The subjunctive mood figures prominently in the grammar of the Romance languages, which require this mood for certain types of dependent clauses. This point commonly causes difficulty for English speakers learning these languages.
FIXME: what is this mood?
The optative mood expresses potential events or wishes and has other uses that overlap with the subjunctive mood. Few languages have an optative as a distinct mood; one that does is [Amcient Greek]?.