Like, prepositions, pronouns and determiners, conjunctions are a small and stable class of terms. There about 30 words used as conjunctions in English.

Conjunctions have the function of stringing together two or more grammatical elements together. Conjunctions are used to join multiple clauses into a single sentence and a string of adverbials in a single statement. They are also used in noun-terms, adjective-terms and adverb-terms to build a list of ideas.

Traditionally, conjunctions are divided into three types: co-ordinating, subordinating and correlating conjunctions. Underlying this distinction is another between a main clause and a subordinate clause. The problem I think with this distinction is that it is somewhat circular. How can you tell whether a clause is a main clause or a subordinate clause? By checking whether it is introduced by a co-ordinating or a subordinating conjunction. What is the difference between a co-ordinating conjunction and a sub-ordinating conjunction? Co-ordinating conjunctions introduce main clauses and subordinating conjunctions introduce subordinate clauses.

There are a small number of coordinating conjunctions:

And a larger number of sub-ordinating conjunctions.

There are also a number of compound sub-ordinating conjunctions, similar to compound prepositions.

The distinction is perhaps a consequence of English’s Germanic roots. In German, main clauses (Hauptsätze), introduced by co-ordinating conjunctions, and subordinate clauses (Nebensätze), introduced by subordinating conjunctions, have different syntax. There is one set of conjunctions that introduce main clauses and a separate set that introduce subordinate clauses. The two sets map to their cognate English terms.

There are no syntactic differences between a main clause and a subordinate in English. Nor is there really a semantic difference. It’s sometimes argued that subordinate clauses cannot stand on their own but any clause is a complete statement and can stand independently syntactically. Semantically, this may not be the case but the same is true of main clauses.

Where the distinction is more important is in the use of conjunctions to link other grammatical elements. Subordinating conjunctions can be used to build multiple clauses into a sentence but cannot be used to build lists or link together strings of adjectives and adverbs. Only the coordinating conjunctions and, or, nor, but and yet can be used in these constructions.

A third group of conjunctions are called correlating conjunctions that are used to build pairs (see section 7.1). These are phrases introduced by an adjective that function as logical operators and are used to join multiple clauses into sentences and also to build lists.