An ELT Glossary : Conjunction

A conjunction is a type of connective expression which is able to (though doesn't necessarily) join two items of the same grammatical class or level together within a sentence. 

Conjunctions may be either co-ordinating or subordinating :

1) Co-ordinating conjunctions (also called co-ordinators). These may join  (a) single    
    words of the same word class, (b) phrases or (c) clauses. Eg : 

    a)  He worked slowly but surely. (adverb + adverb)
    b)  I bought some gloves and a new pair of boots.  (noun phrase + noun phrase)
    c)  I must leave now, or I'll be late  (clause + clause)

    They are also sometimes found indicating the semantic connection between two     
    clauses, but with the second clause starting a new sentence.
    d)  (From Barack Obama's "Yes we can" speech): Senator McCain fought long and hard in this 
         campaign. And he's fought even longer and harder for the country he loves." (sentence +       

There are various syntactic conditions which allow us to identify co-ordinating conjunctions :

a. They always occur between the two co-ordinated items. It may be possible to change the order of the items, but the co-ordinator remains in the middle :
It was raining and the wind was blowing strongly / The wind was blowing strongly and it was raining.

b. It cannot be preceded by another conjunction. 

c. Words can be ellipted from one part of the coordinated phrase/clause if they are included in and therefore retrievable from the other.
They were laughing and joking. = They were laughing and they were joking.
He climbed up and down the tree several times = He climbed up the tree and he climbed down the tree several times.

d. It can link subordinate clauses : Although it sounded interesting, and although we would have liked to go, we didn't have time.

e. More than two items can be linked, and when this happens, all but the last instance of the co-ordinator can be omitted : It was a cold, grey and rainy day. = It was a cold (and) grey, and rainy day

And, but and or meet all these conditions and are therefore "pure" co-ordinators. Other words (yet, for, then, so), which have traditionally been considered to be co-ordinators, meet some but not others. Eg yet, so and then do not meet criterion 2 as they can (and should) be preceded by another co-ordinator : 
He arrived late, and yet did more than anyone else
He arrived late, and so missed the train.
He arrived late, and then wasted more time making coffee.

Quirk et al (1972) therefore class them simply as conjuncts.

Similarly, For  does not meet criterion 3 regarding ellipsis :
He was not worried about time, for he knew the schedule could be changed - the word he is obligatory in the second clause.

For is often classed as a subordinating conjunction, yet does not meet the criterion for subordinating conjunctions stated below - ie that clause position can be reversed :
*For he knew the schedule could be changed, he was not worried about time
Compare this with because, which is a true subordinating conjunction   

2) Subordinating Conjunctions (also called subordinators). These join a main (or 
    independent) clause and subordinate clause into one sentence.  More than one             
    subordinate clause may be linked to the main clause, eg

    I knew immediately that he had been there because he'd left his keys on the table.
    Main clause              Subordinate clause       Subordinate clause

    The conjunctions are part of the subordinate clause, and if the clause changes       
    position, the subordinating conjunction will move with it:

    I didn't go, although it would have been interesting.
    Although it would have been interesting, I didn't go.

   Examples of subordinating conjunctions include because, whereas, although, unless, if.                                                                         

B. Conjunctions may be :

     i)   simple - ie composed of single words (as in the examples above) 

     ii)  compound - ie composed of more than one word - eg as soon as : Can you call me 
          as soon as he arrives?  (Compound subordinating conjunction)

     iii) correlative - ie composed of two separate elements. Eg: either/or
          I think I'll buy either the blue one or the green one.  (Co-ordinating correlative      

Further reading

Parrott, M. Grammar for English Language Teachers, Cambridge

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