The Classification of Words

THE CONJUNCTION
§ 353. The conjunction is a part of speech characterized by the following features.

1. 
   Its lexico-grammatical meaning of 'relations between
   substances, actions, properties, situations, etc.'.
   
2. 
   Its peculiar combinability. As a rule, a conjunction
   connects two similar units: words of a similar type or clauses.
   
3. 
   Its function of a linking word.
   

§ 354. Conjunctions are not characterized by any grammat­ical categories or typical stem-building elements. As to their stem-structure conjunctions are, as usual, divided into simple (and, but, or, that, till, if, etc.), derivative (until, unless, because, provided, etc.), compound (although, whereas, etc.) and composite (as if, in order that, as soon as, either ... or, neither ... nor, etc.).
A variety of composite conjunctions is the group of the so-called correlative conjunctions which go in pairs: both... and, either ... or, no sooner ... than, etc.
§ 355. Many conjunctions are homonymous with adverbs and prepositions (after, since, before), pronouns (that, so, neither), participles (supposing, provided).
§ 356. The lexico-grammatical meaning of conjunctions is an abstraction from their lexical meanings. The latter are also very general, abstract and rather weak. Therefore conjunctions can be treated as semi-notional words (see § 50), though not as form-words *, since they are not devoid of content.
§ 357. As regards the nature of the relations they serve to express conjunctions are usually divided into two subclasses: coordinating (and, or, both ... and, etc.) and subordinating (if, that, as soon as, etc.) conjunctions.
¹ See M. Ganshina, N. Vasilevskaya, op. cit., p. 252.
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The former connect syntactical units which are equal in rank, discharge the same functions. The latter are used to show the dependence of one unit on another.
Cf. This is crystal clear and the government admits it. (Daily Worker).
I f they did so, their complete fare would be refunded, (Daily Worker).
§ 358. The combinability of coordinating conjunctions is bilateral like that of prepositions. But there are essential differences.

1. 
   The right-hand combinability of prepositions is stable,
   that of conjunctions is variable.
   
2. 
   With prepositions there is no correlation between the
   right-hand and the left-hand connections. With conjunctions
   it is different. A conjunction usually connects a noun with
   a noun (or pro-noun), a verb with a verb, a clause with a
   clause. In this sense a conjunction connects homogeneous
   elements, while a preposition mostly connects heterogeneous
   elements'.
   
3. 
   A preposition cannot introduce a clause without a con-
   nectiv.e word, as a conjunction does. Cf. It depends on when
   (where, how, why) he does it, not * It depends on he does it.
   

§ 359. The combinability of subordinating conjunctions is somewhat different from that of coordinating ones.
1. Subordinating conjunctions connect mostly clauses, not
words.
Compare, however, cases like Though young, he is a skilled worker. He is skilful though young.
2. Very often a subordinating conjunction begins a ^cen-
tence, so that it precedes both clauses it connects.
Cf. They would hold a sitdown strike и n I e s s the P. and 0. Company arranged for them to continue their journey by sea. (Daily Worker)
Unless the P. and 0. Company arranged for them to continue their journey by sea, they would hold a sitdown strike.
§ 360. The division of conjunctions into coordinating and subordinating ones is chiefly based on their lexio-al meanings and the types of units they connect.
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Coordinating conjunctions are used in simple and in compound sentences. In other words, they are used to connect both words and clauses equal in rank. It is to be observed, however, that some coordinating conjunctions never occur in simple sentences (so, for), while others (both ... and, as well as) are used only in simple sentences ¹.
Subordinating conjunctions uniting clauses not equal in rank, naturally₂ occur in complex sentences as a meanj of connecting subordinate clauses with their main clauses. There i§ a smalj group (that, if, whether) introducing the so-called noun-clauses, i. e. subject, object, predicative and appositive clauses. Most subordinating conjunctions introduce adverbial clauses of time, place, condition, purpose, result, cause, condition, comparison, etc.