A conjunction joins two or more parts of a sentence and expresses how the parts are related. There are two main types of conjunctions: coordinating and subordinating.
The principal coordinating conjunctions are and, but, for, nor, and or. They join sentence elements that have equal weight and the same grammatical character. They can connect words or entire phrases and clauses.
John had eggs, bacon, and toast for breakfast.
In the sentence above, and joins the words for the breakfast items. It also shows how they are related. John had all three things for breakfast, not merely one or two of them.
The train had to make several extra stops, but Ming still managed to arrive at work on time.
In the sentence above, but joins two independent clauses and shows how they relate to each other. Here but means “on the contrary,” or “notwithstanding.” The fact that the train had to make extra stops might have made Ming late. He got to work on time despite the delay.
Sometimes coordinating conjunctions are preceded by correlatives. Examples are both . . . and, neither . . . nor, and not only . . . but also. These correlative conjunctions heighten the parallelism of the units that they join. The units joined should be in the same grammatical form.
The film was both loved by the public and panned by critics.
In the sentence above, the correlative conjunction both . . . and joins two verb phrases that are in the same form.
Subordinating conjunctions, unlike coordinating ones, do not join words or phrases that have equal weight. They often introduce adverbial clauses that modify or qualify the main clause or the main verb of the sentence.
They drove until they ran out of gas.
In the sentence above, the phrase ran out of gas works as an adverb modifying the verb drove, and until is the subordinating conjunction. Other common subordinating conjunctions are after, although, as, as if, as long as, because, before, since, so that, and while.
Some words that work as subordinating conjunctions can also be prepositions. But prepositions always introduce a noun or noun equivalent, whereas subordinating conjunctions introduce a subject-verb combination. In the sentence They drove until dawn, until is a preposition.
The word that, when used as a subordinating conjunction, introduces a noun clause that does not modify the main clause or its verb.
I heard that you bought a new car.
In the sentence above, that is a subordinating conjunction. It does not modify heard, though. Rather, it answers the question, What did I hear?