Means of expressing modality in Modern English

CHAPTER II. GENERAL CHARACTERISTIC OF MODAL VERBS
The group of modal verbs includes a small number of the verbs which are distinguished from all verbs with a number of characteristics in value, use and grammatical forms. These verbs have no actual verbal grammatical category (a look, a temporary denotation of pledge); they can have only the inclination and time forms which are indicators of a predicate. Owing to this fact, and also owing to absence of non-predicative forms (an infinitive, a gerund, participles), modal verbs stand on the periphery of verbal system in the English language.
They designate opportunity, ability, probability, need of commission of the action expressed by a notional verb. As they express only modal relation, instead of action, they as a separate sentence part are never used. Modal verbs are always combined only with an infinitive, forming with it combinations which in the sentence is a complete modal predicate. [10,137]
Modal verbs are deficient verbs (Defective Verbs), because they do not have any forms that have other verbs. Lack of inflected-s in 3-person singular of the present tense is historically saying: modern forms of this time were the forms of the past tense, and 3 of the singular past tense was not personal.
Modal verbs must, should - ought, will-would, can-could, may-might, need can express various shades of the assumption. Scientists assume that modal verbs express objective reality, while parenthesizes – subjective. It is possible to assume that verbs can and may specialize on transfer of possible, alleged actions, and verbs of must, should, might, besides value of obligation, transfer also alleged, probable actions, with value of parenthesizes, such as perhaps, possibly, probably, certainly. When modal words and parenthesizes are used at the same time, in such cases we deal with synonymous designs. [10,154]
In the offer modal verbs are always combined with an infinitive (perfect and not perfect), forming with it one combination which is called a compound modal predicate. As separate sentence parts modal verbs are not used.