We use language to interact with others. In conversations we try to fulfill various purposes: to invite, to order, to confirm, to describe, to evaluate, to sympathize, to agree, to contradict, and so on. However, all these unlimited purposes fall into two fundamental categories: giving or demanding. What we give or demand is either goods-and-services or information. Generally, giving goods-and- services through language is offer; giving information is statement. Demanding goods-and-services is command; demanding information is question. These basic interpersonal exchanges are associated with three types of syntactic forms: statements are mostly expressed by declarative sentences; questions by interrogative sentences; commands by imperative sentences. Offering, as a speech functions, can be expressed by any of these three types (I will give you the book. Would you take the book? Take the book, please.). This observation has already been made in traditional grammar. The unique contribution by systematic-functional grammar is the observation that only a particular element of syntactic structure, called mood, realizes the interaction between the speaker (writer) and hearer (reader). Consider:
(58) ---The president has turned down the proposal, hasn't he?
---Yes, he has.
No, he hasn't.
Oh, has he?
He hasn't, but he will.
In this communicative interaction, only the subject and the auxiliary are manipulated back and forth while the remainder of the structure is not affected. The remainder is called Residue.
Mood is a syntactic constituent made up of the Subject and the Finite. The subject is the first noun phrase in a statement. It is replaced by its corresponding personal pronoun in the question tag if the head of the noun phrase is a noun. The finite is a verbal operator that expresses tense (e.g. is, were, has, had) or modal meaning (e.g. can, should, must) as well as positive or negative form. When the verb is in simple present or past tense, active voice and positive form, the finite is ‘fused’ with the lexical verb (e.g. gave, works). In tag questions the finite (did, does) will appear in the tag (e.g. He left here, didn’t he? You know, don't you ?). Mood (The subject and the finite) and Residue are shown below.
The order of the subject and the finite realizes the speaker's choice of declaratives, interrogatives or imperatives. The realization can be shown by the following figure:
The Finite expresses not only tense but also polarity and modality. Any finite expresses polarity because it is either positive or negative (However, polarity may be expressed by never, hardly, etc. which are not finite. In this case, the finite is grammatically positive, but the meaning is negative). The grammatical form of the finite is polarized. Nevertheless, there are, semantically, intermediate stages between the two polarities ‘yes’ and ‘no’. The semantic space between ‘yes’ and ‘no’ is expressed by modality. The following figure illustrates this concept.
Modality can be categorized into modalization and modulation according to the distinction of speech roles that was discussed at the beginning of this section. If the speaker is providing information by the utterance, the modality relates to how valid the information is in terms of probability (how likely it is) or usuality (how frequent it is). The probability modal space is expressed by possible, likely, probable, certain, sure, etc. The usuality modal space is expressed by sometimes, often, usually, always, etc. If the speaker speaks of goods-and-services, the modality relates to how confident the speaker can be in the eventual success of the speech act. In commands the modal space concerns the degree of obligation on the person to carry out the commands, while in offers it regards the degree of inclination of the speaker to fulfill the offer. The two types of modality and their sub-categories can be summarized in the following manner:
