Halliday & Matthiessen (2014: 103):
… here, let’s is clearly the unmarked choice of Theme.
Thus in both kinds of interrogative clause the choice of a typical ‘unmarked’ thematic pattern is clearly motivated, since this pattern has evolved as the means of carrying the basic message of the clause. Hence there is a strong tendency for the speaker to choose the unmarked form, and not to override it by introducing a marked Theme out in front. But marked Themes do sometimes occur in interrogatives.
If the WH- word is, or is part of, a nominal group functioning as Complement in a prepositional phrase, this nominal group may function as Theme on its own, e.g. what in what shall I mend it with?, which house in which house do they live in? If the WH- element serves in a projected clause, it may serve as the Theme of the projecting clause, as in Who do you think pays the rent?, which is the interrogative version of you think somebody pays the rent.
Interrogative clauses, therefore, embody the thematic principle in their structural make-up. It is characteristic of an interrogative clause in English that one particular element comes first; and the reason for this is that that element, owing to the very nature of a question, has the status of a Theme. The speaker is not making an instantial choice to put this element first; its occurrence in first position is the regular pattern by which the interrogative is expressed. It has become part of the system of the language, and the explanation for this lies in the thematic significance that is attached to first position in the English clause. Interrogatives express questions; the natural theme of a question is ‘I want to be told something’; the answer required is either a piece of information about an element of the clause or an indication of polarity. So the realisation of interrogative mood involves selecting an element that indicates the kind of answer required, and putting it at the beginning of the clause.
In a WH- interrogative, which is a search for a missing piece of information, the element that functions as Theme is the element that requests this information, namely the WH- element. It is the WH- element that expresses the nature of the missing piece: who, what, when, how, etc. So in a WH- interrogative the WH- element is put first no matter what other function it has in the mood structure of the clause, whether Subject, Adjunct or Complement. The meaning is ‘I want you to tell me the person, thing, time, manner, etc.’.
In a yes/no interrogative, which is a question about polarity, the element that functions as Theme is the element that embodies the expression of polarity, namely the Finite verbal operator. … but, since that is not an element in the experiential structure of the clause, the Theme extends over the following Subject as well.
There is one sub-category of declarative clause that has a special thematic structure, namely the exclamative. These typically have an exclamatory WH-element as Theme … .
The ‘most marked’ type of Theme in a declarative clause is thus a Complement: for example this responsibility in this responsibility we accept wholly. This is a nominal element that, being nominal, has the potentiality of being Subject; which has not been selected as Subject; and which nevertheless has been made thematic. Since it could have been Subject, and therefore unmarked Theme, there must be very good reason for making it a thematic Complement – it is being explicitly foregrounded as the Theme of the clause.*
* It is also likely to be given the status of New information within its own unit of information. At the same time, some element other than the Complement will be a candidate for the status of New within the Rheme of the clause, as in this they should refuse.
Marked Adjunct and Complement Themes are followed by the Subject in Modern English — a[n] historical departure from the general principle in Germanic languages that the Theme is followed by, and thus marked off by, the Finite in a declarative clause. The general exception to this departure in Modern English is a clausal negative item as Theme — an Adjunct or Complement with a negative feature that pertains to the clause.* Such negative Themes are followed by the Finite.
* This applies to circumstantial Adjuncts with a negative feature, e.g. nowhere as a locative Adjunct; and it also applies to modal Adjuncts with a negative (or quasi-negative) feature, e.g. never, hardly.
A Theme that is something other than the Subject, in a declarative clause, we shall refer to as a marked Theme. The most usual form of marked Theme is an adverbial group … or prepositional phrase … functioning as Adjunct in the clause. Least likely to be thematic is a Complement, which is a nominal group that is not functioning as Subject — something that could have been a Subject but is not … . Sometimes even the Complement from within a prepositional phrase functions as Theme … .
In a declarative clause, the typical pattern is one in which Theme is conflated with Subject; … We shall refer to the mapping of Theme on to Subject as the unmarked Theme of a declarative clause. The Subject is the element that is chosen as Theme unless there is good reason for choosing something else.
Every free clause selects for mood. … A free major clause is either indicative (giving or demanding information) or imperative (demanding goods-&-services) in mood; if indicative, it is either declarative (giving information) or interrogative (demanding information); if interrogative, it is either ‘yes/no’ interrogative or ‘WH-’ interrogative.
MOOD is the major interpersonal system of the clause; it provides interactants involved in dialogue with the resources for giving or demanding a commodity, either information or goods-&-services – in other words, with the resources for enacting speech functions (speech acts) through the grammar of the clause: statements (giving information), questions (demanding information), offers (giving goods-&-services), and commands (demanding goods-&-services).
The thematic equative actually realises two distinct semantic features, which happen to correspond to the two senses of the word identify. On the one hand, it identifies (specifies) what the Theme is; on the other hand, it identifies it (equates it) with the Rheme. The second of these features adds a semantic component of exclusiveness: the meaning is ‘this and this alone’.
In a thematic equative, all the elements of the clause are organised into two constituents; these two are then linked by a relationship of identity, a kind of ‘equals sign’, expressed by some form of the verb be. …
A thematic equative (… a ‘pseudo-cleft sentence’ …) is an identifying clause which has a thematic nominalisation in it. Its function is to express the Theme–Rheme structure in such a way as to allow for the Theme to consist of any subset of the elements of the clause. This is the explanation for the evolution of clauses of this type: they have evolved, in English, as a thematic resource, enabling the message to be structured in whatever way the speaker or writer wants.
… if a clause is organised into two information units, the boundary between the two is overwhelmingly likely to coincide with the junction of Theme and Rheme.
// However // information boundaries are highly likely between textual and experiential Themes //
// Actually // information boundaries are highly likely between interpersonal and experiential Themes //
Some grammarians have used the terms Topic and Comment instead of Theme and Rheme. But the Topic–Comment terminology carries rather different connotations. The label ‘Topic’ usually refers to only one particular kind of Theme, the ‘topical Theme’; and it tends to be used as a cover term for two concepts that are functionally distinct, one being that of Theme and the other being that of Given. It seems preferable to retain the earlier terminology of Theme–Rheme. In the generative linguistic literature, Gruber (1976: 38) introduced the term ‘theme’ in an experiential (rather than textual) sense for a kind of participant role, a ‘theta role’ in generative terms. In work drawing on Fillmore’s (1968) ‘case grammar’, the term ‘theme’ has also been used as a label for deep case, or semantic case. In a different context, ‘theme’ is also used as the name of a stratum in verbal art: see Hasan (1985b: 96).
The structure which carries this line of meaning is known as thematic structure. … a form of organisation whereby it fits in with, and contributes to, the flow of discourse.
… we introduced the notion of a clause as a unit in which meanings of three different kinds are combined. Three distinct structures, each expressing one kind of semantic organisation, are mapped on to one another to produce a single wording.
It is the segmental kind of structure, with clearly separated constituent parts organised into a whole, that has traditionally been taken as the norm in descriptions of grammar; the very concept of ‘structure’, in language, has been defined in constituency terms. This is partly because of the kind of meaning that is expressed in this way: experiential meaning has been much more fully described than meaning of the other kinds. But there is also another reason, which is that constituency is the simplest kind of structure, from which the other, more complex kinds can be derived; it is the natural one to take as prototypical — in the same way as digital systems are taken as the norm from which analogue systems can be derived, rather than the other way round.
The general principle of discreteness means that each structural unit has clearly defined boundaries. But while this kind of segmental organisation is characteristic of the clause as representation, the clause in its other guises – as message, and as exchange – departs from this prototype. In its status as an exchange, the clause depends on prosodic features — continuous forms of expression, often with indeterminate boundaries; while in its status as message it tends to favour culminative patterns — peaks of prominence located at beginnings and endings.
The general principle of hierarchy means that an element of any given rank is constructed out of elements of the rank next below. This is a feature of the constituent hierarchy made up of units and their classes: clause, verbal group, and so on. But the configurations of structural functions show further ramifications of this general pattern. Thus, in the clause as exchange there is slightly more layering in the structure, while in the clause as message there is rather less.
The general principle of exhaustiveness means that everything in the wording has some function at every rank. But not everything has a function in every dimension of structure; for example, some parts of the clause (e.g. interpersonal Adjuncts such as perhaps and textual Adjuncts such as however) play no rôle in the clause as representation.
So far, we have referred to constituent structure as if it was something uniform and homogeneous; but as we embark on the detailed analysis of clause structures this picture will need to be modified. The model of constituent structure that we presented — the rank scale — is the prototype to which all three metafunctions can be referred. But the actual forms of structural organisation depart from this prototype, each of them in different ways.
… these three kinds of meaning run throughout the whole of language, and in a fundamental respect they determine the way language has evolved. They are referred to in systemic accounts of grammar as metafunctions, and the concept of ‘metafunction’ is one of the basic concepts around which the theory is constructed.
The significance of any functional label lies in its relationship to the other functions with which it is structurally associated. It is the structure as a whole, the total configuration of functions, that construes, or realises, the meaning. … It is the relation among all these [functions] that constitutes the structure.
A clause has meaning as a representation of some process in ongoing human experience; the Actor is the active participant in that process. It is the element the speaker portrays as the one that does the deed.
A clause has meaning as an exchange, a transaction between speaker and listener; the Subject is the warranty of the exchange. It is the element the speaker makes responsible for the validity of what he is saying.