Conjunctions
A "conjunction" is used to show the relationship between two ideas in a sentence. Sometimes the ideas are expressed as single words or phrases--as simple as a noun or an adjective. (Note that conjunctions used in this way require the use of "parallelism," where the same part of speech must occur on both sides of the conjunction.)
This course is hard but not impossible.
(hard and impossible are adjectives)
I'm taking English and biology this semester.
(English and biology are nouns.)
Often, conjunctions connect more complex ideas, expressed in the form of clauses. (A clause can be loosely defined as a group of words having its own subject and verb.)
When showing the relationship between clauses, conjunctions are usually divided into two sub-classes. These are "coordinating conjunctions" and "subordinating conjunctions."
Coordinating Conjunctions
Coordinating conjunctions are defined as conjunctions which show the relationship between two ideas, or clauses, of equal importance.
We plan to go to the theater this evening, but we may be late.
You could finish out the year, or you could take a leave of absence.
I plan to retake the accounting course and hopefully do better than last time.
Subordinating Conjunctions
Subordinating conjunctions are defined as conjunctions which show the relationship of a less important idea to a more important idea.
When you go to the lab, you'll need to take your calculator.
You'll need your calculator because we will be working on some formulas.
If you don't have your calculator, you won't be able to participate in the class.
Sometimes, the function of a subordinating conjunction can be expressed by a phrase, rather than a single word. Such phrases often end with the word that:
By the time (that) you read this letter, I will be on a plane to Las Vegas.
I will consider giving you a raise provided that you promise to work harder.
The start-up company hired a lobbyist so that they could get local tax relief.
Conjunctive Adverbs
A conjunctive adverb has a similar function to a conjunction in that it is also used to show the relationship between two clauses. However, usually the punctuation is different, and grammarians don't typically divide conjunctive adverbs into coordinating and subordinating sub-classes.
His behavior in class is not the best; moreover, his work so far is very poor.
Getting a degree in engineering is pretty tough; however, I think you could do it.
Conjunctive adverbs may also be used to show the relationship between ideas across sentence boundaries.
There are lots of reasons why Buffalo is losing population. For example, many of the old manufacturing industries have fallen on hard times.
In an Ironman race, you start with a two-mile swim. Next, you cycle 112 miles. Then you run a marathon.
Prepositions
While conjunctions and conjunctive adverbs typically show the relationship between two clauses, prepositions typically show connections between ideas within clauses.
The internet company finally made a profit after two years of losses
The sailing class took the boats out despite the lack of wind.
Some prepositions are multi-word prepositions (for example, in spite of):
Marcia had to take a leave of absence because of illness.
In contrast to most people on my dormitory floor, I like to go to bed early.
In all of the examples above, the preposition is followed by a noun phrase, which is called the "object" of the preposition. In the last two examples above, the objects of the prepositions are illness and most people on my dormitory floor.
While the object of a preposition is generally a noun phrase, prepositions can also be followed by a "gerund" instead of a noun phrase. A gerund is a verb + ing form that behaves in many ways like a noun phrase. In the following two examples, the objects of the (multi-word) prepositions are the gerunds studying and running.
In spite of studying all night, I still did badly on the midterm.
A triathlon involves swimming and cycling in addition to running.
It is important to recognize that sometimes the structure conjunction + clause can be abbreviated to preposition + gerund. Compare the following sentence pairs:
It's not easy to communicate in sign language while you're driving.
It's not easy to communicate in sign language while driving.
He had a heart attack because he worked too hard.
He had a heart attack because of working too hard.
In the first pair of sentences, while in the first sentence is a conjunction that introduces the clause while you're driving; while in the second sentence is a preposition following by the gerund driving. In the second pair of sentences, because in the first sentence is a conjunction that introduces the clause because he worked so hard; because of in the second sentence is a preposition following by the gerund working.