On the one hand

§ 31. Let us compare the following columns of words:
teach — teacher real — realize
work — worker national — nationalize
lead — leader , individual — individualize
write — writer natural — naturalize
read — reader industrial — industrialize
The words of column 1 and those of column 2 belong to different classes of lexemes. The same is true of the words of the last two columns.
These classes differ not only in their lexico-grammatical meanings (morphemes), but in some grammatical properties as well: different opposemes, paradigms, etc. Such classes of lexemes have been called parts of speech for over 2000 years. Therefore we dare not change the name. But we must remember that classes of units exist only in the system of a language. In speech we come across combinations of individual represent­atives of various classes.
Parts of speech are the largest word-classes that may contain endless numbers of word-groups such as lexemes or grammemes.
It is certainly easier to survey a limited number of parts of speech than an ocean of lexemes or grammemes. Therefore it has been a long-standing tradition to study the properties of words within the framework of parts of speach. The chapter headings of- a book on morphology are usually the names of the parts of speech. We shall adhere to the tradition and after a chapter on the general criteria for dividing English words into parts of speech we shall analyse them one by one.
THE COMBINABILITY OF WORDS
§ 32. As already mentioned (§ 2), only those combinations of words (or single words) which convey communications are sentences — the object of syntax. All other combinations of words regularly formed in the process of speech are the object
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of morphology as well as single words. Like separate words I hey name things, phenomena, actions, qualities, etc., but in a complex way (cf. manners and table manners, blue and dark blue, speak and speak loudly). Like separate words they serve as building material for sentences.
In English the demarcation line between certain types of words and certain types of word-combinations is vague. Com­pare, for instance, the words blackboard, head-master, brother-in-law and the word-combinations black board, head waiter, brother in arms.
Some combinations' of words in English have become so stable and their meaning so fused and so different from the meanings of their components that they are called phraseolo­gical fusions, or idioms. Retaining the forms of combinations of words, they function like single words and may be regarded as word equivalents, units of the language system, alongside of words.
E. g. Under the rose = in secret, secretly, Once in a blue moon — rarely, or never.
Other combinations of words are more or less freely formed in numerous acts of speech. For instance, the word his may be freely combined with the words face, book, love, absence, etc. Grammar mostly studies the relations between the words of free combinations, whereas lexicology analyses phraseological units.
§ 33. The combinability of words is as a rule determined by their meanings, not their forms. Therefore not every se­quence of words may be regarded as a combination of words. In the sentence Frankly, father, I have been a fool neither frankly, father nor father, I ... are combinations of words since their meanings are detached and do not unite them, which is marked orally by intonation and> often graphically by punctuation marks.
On the other hand, some words may be inserted between the components of a word-combination without breaking it.
Compare,

1. 
   read books
   
2. 
   read many books
   

c) read very many books.
In case (a) the combination read books is uninterrupted. In cases (b) and (c) it is interrupted, or discontinuous (read... books).
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§ 34, The combinability of words depends on their lexical, grammatical and lexico-grammatical meanings. It is owing to the lexical meanings of the corresponding lexemes that the word wise can be combined with the words man, act, saying and is hardly combinable with the words milk, area, outline.
The lexico-grammatical meanings of -er in singer (a noun) and -ly in beautifully (an adverb) do not go together and pre­vent these words from forming a combination, whereas beautiful singer and sing beautifully are regular word-combina­tions.
The combination * students sings is impossible owing to the grammatical meanings of the corresponding grammemes.
Thus one may speak of lexical, grammatical and lexico-grammatical combinability, or the combinability of lexemes, grammemes and parts of speech.
§ 35. The mechanism of combinability is very complicated. One has to take into consideration not only the combinability of homogeneous units, e. g. the words of one lexeme with those of another lexeme. A lexeme is often not combinable with a whole class of lexemes or with certain grammemes. For instance, the lexeme few, fewer, fewest is not combinable with a class of nouns called uncountables, such as milk, information, hatred, etc., or with members of 'singular' grammemes (i. e. grammemes containing the meaning of* 'singularity', such as book, table, man, boy, etc.).
The 'possessive case' grammemes are rarely combined with verbs, barring the gerund. Some words are regularly combined with sentences, others are not. All this will be dwelt on in the corresponding parts of this book.