Б. С. Хаймович, Б. И. Роговская теоретическая грамматика английского языка

THE MODAL WORDS
(MODALS)

§ 329. As a part of speech the modals are characterized by the following features:

1. Their lexico-grammatical meaning of 'modality'.
2. Their negative combinability.
3. Their functions of parenthetical elements and sentence-words.

§ 330. 'Modality' as a linguistic term denotes the relation of the contents of speech to reality as viewed by the speaker. When describing the meaning of 'modality' in the small group of modal verbs we are in fact dealing with lexical 'modality'. The 'modality' of the indicative, subjunctive and imperative moods is grammatical 'modality'. Now we are dealing with the meaning of 'modality' uniting a part of speech. This is lexico-grammatical 'modality'.

Modal words indicate whether the speaker is sure that the contents of his utterance correspond to reality, or he doubts it, or he regards it as something possible, probable, desirable, etc. Accordingly, modal words can be divided into several groups.
a) Those which denote various shades of certainty: certainly, surely, of course, no doubt, assuredly, undoubtedly, indeed, really, etc.
b) Those which denote various degrees of probability: maybe, perhaps, possibly, probably, etc.
c) Those which denote various shades of desirability (undesirability): happily, luckily, fortunately, unhappily, etc.

§ 331. The relatively negative combinability of modal words manifests itself in various ways.

a) They are almost never used as adjuncts to some head­word.
b) They but seldom function as head-words to some ad­juncts, mostly adverbs of degree like very, quite, most, etc.
E. g... whom most probably they were compelled to respect. (Dreiser).
c) Their isolatability (§ 6) is greater than that of other words. They very often make response sentences.
E. g. But you can take a carpet to Caesar in it if I send one? — A ssuredly. (Shaw).

§ 332. Functioning as a parenthetical element of a sentence, a modal word is usually connected with the sentence as a whole.

E. g. Perhaps I shall never pray again. (Shaw).
Apparently, they were fully prepared for the coming of the visitors from England. (Tracy).
But sometimes it may be connected with a part of the sentence only.
E. g. We worked that land for т а у b e a hundred years (Daily Worker).
§ 333. The usage of modals depends upon the type of sentence. They are found almost exclusively in declarative sentences, very rarely in interrogative and almost never in imperative sentences.
According to S. E. Kagan there are 256 modal words in The Man of Property by J. Galsworthy. 250 of them are in declarative sentences, 6 in interrogative ones and none in imperative sentences. This fact can easily be accounted for. Interrogative and imperative sentences are used not in order to express one's knowledge of reality with various degrees of certainty or doubt. They are means of urging somebody else to say something or do something.