One of the ways that verbs can be classified in a clause is by their mood. This term traditionally encompasses the indicative, interrogative, imperative and subjunctive. Just as in real life people can be in different moods (frame of mind), the grammatical moods can be considered as expressing different frames of thought.
This is the default and by far the most common mood. It expresses information about real events or actions, states and processes.
- I played tennis yesterday (action).
- She was very tired after work (state).
- Water boils at 100°C. (process).
The example sentences in the tense selector article are in the indicative mood.
The interrogative is the questioning mood. For example:
- Did you play tennis yesterday?
- How did she feel after work?
- At what temperature does water boil?(process).
The imperative is the mood used to give commands or instructions. It uses the base form of the verb. In comparison with the subjunctive, there is nothing difficult for English learners about this mood.
- Give it to me.
- Stop that, please.
- Use a pen to write your answers.
- Somebody answer the phone.
- Don't ask me again.
The subjunctive mood expresses, among other things, counterfactuals, hypotheses, recommendations and importance. The subjunctive can be recognised by inflections that differ from indicative inflections: I were..., he take..., they be..., etc.
- If I were you, I'd say sorry (counterfactual).
- If he were to apologize, she would forgive him (hypothesis).
- The doctor recommends that he take the medication before breakfast (recommendation).
- It is important that she be at the airport before 6am (importance).
The third and fourth sentences above are examples of the mandative subjunctive. This is a formal usage that is more common in American English than in British English In British English it is more likely that the indicative is used or a circumlocution with a modal verb.
- It is important that she is at the airport before 6am.
- It is important that she should be at the airport before 6am. .
The subjunctive also lives on in formulaic expressions such as:
- God save the king.
- Be that as it may.
- Heaven forbid.
- Far be it for me... .
As in other areas of grammar, there is no consensus on what constitutes a mood in English. Some modern grammarians, for example, exclude the subjunctive. Others exclude the interrogative.
In my experience, however, learners are more interested in meaning, usage and grammaticality than in terminology and classification. Those wish to inform themselves on the latter are recommended to consult The Cambridge Grammar of the English language.