Compoundare formed syntactically, i.e. from set expressions acquiring gradually properties of a word. The set expressions may be of various types. Sometimes, these are of the “preposition + noun” structure. One of the conditions that enable this word combination to transform into a modal word is, firstly, meaning of subjective evaluation, and, secondly, adverbialization of this word combination.
Some scholars refer tosuch word combinations as for certain, for sure, in truth, in fact, etc. on the grounds that these are able to express a number of modal meanings. Other linguists doubt whether it is justified, since (1) meaning of subjective evaluation of these expressions is not their main one; (2) components of the set expressions retain their lexical meaning, which results in the use of attributes (cf. for dead sure, in all truth, in actual fact) that prove that the expressions have not yet been established as a morphological unity; (3) the set expressions are synonymous with same-root (for certain – certainly, for sure – surely, in truth – truly), which prevents these set expressions from entering the class of modal words.
Another morphologicalis made up of words derived from verbs (maybe, meseems). These are also formed syntactically by of their components. Their peculiarity lies in that the modal meaning of one of the components comes to be projected onto the whole compound. Morphologically, these modal words are compounds consisting of two stems.
Syntactically modal words are characterized by the function of parenthesis, used both within a syntactic structure and as an independent word-sentence. Used as parenthesis, a modal word refers either to the content of a sentence on the whole or to some its part. When a modal word covers the whole sentence, it is used either in its beginning or in the end. In other cases, modal words are placed in immediate proximity to the structure to which they refer.
In conclusion, it is necessary to mention such words as (un)luckily, (un)happily, (un)fortunately whose categorical status has not been decided on yet. Some linguists regard these words as a specific semantic subclass of modal words – as words that convey the subjective meaning “desirability -undesirability”. Meanwhile other researchers claim that these words have a number of peculiarities strange to modal words. Semantically, they express the speaker’s emotion or volition. Morphologically, such words as (un)luckily may form derivational antonyms by means of the suffix un- and, consequently, are morphologically divisible. Syntactically, words of the (un) luckily-type are less independent, and they also are seldom (if ever) used as word-sentences. All these arguments are put forward to deny these linguistic units the status of modal words. This, however, does not prevent us from noting several properties that the (un)luckily-type words share with modal ones. Take, for instance, the inextricable semantic connection between the logico-rational and the emotional-volitional, as well as the syntactic function of parenthesis. As a result, it is quite natural to conjecture that the (un)luckily-type words represent transition cases between adverbs and modal words.
Thus, modal words in English exhibit a variety of derivational patterns and do not possess any distinct formal properties. Meanwhile, the independence of their meaning and their specific syntactic function proves their status of lexical words, though they are characterized by some specific properties.