Conjunctions vs prepositions [closed]

Question

I am so confused as to the difference between conjunctions ans prepositions. There seems to be no clear line between them. I understand that coordinating conjunctions are always conjunctions, but it seems that subordinating conjunctions can be prepositions also. What are the rules?

Answer 1

In formal English, conjunctions normally introduce full clauses, i.e. a finite verb with arguments. Examples:

and, or, nor, because, that, as, for, while, when, if, before, after

Coordinating conjunctions introduce main clauses; subordinating ones introduce subordinate clauses (clauses that can't stand alone without a main clause).

A coordinating conjunction cannot come immediately after another conjunction, while a subordinating one can:

I love her, and, because of that, I must kill her.

Because is a subordinating conjunction, and a coordinating one.

Subordinate clauses can often be moved to a different place in the sentence, such as from before the main clause to after the main clause, while coordinating clauses cannot.

Some coordinating conjunctions can also be used to coordinate two noun groups, like and and or:

I like fish and trees; I don't have any fish nor trees.

Some subordinating conjunctions with a temporal meaning can be used to introduce a mere participle:

She died while cursing her father.

Ellipsis is always possible, to be found under various circumstances, so you might find conjunctions followed by elliptical clauses too (e.g. lacking the normally required finite verb).

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Prepositions, in the commonly used, traditional terminology, can only introduce nouns or noun groups, which includes gerunds, but not full clauses. Examples:

for, of, with, after, in, past, before

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Some words can be used with different senses, in that they can be used as different parts of speech. For can be a conjunction or a preposition:

I hate him. For he has killed my father.

This curse is for him.

She killed him, before she left.

I attacked him before her.

Answer 2

An easy rule-of thumb: To decide if something is a subordinating conjunction, you have to see if there is a subordinate clause.

We left early though we arrived late.

Main clause: we left early

Subordinate clause: we arrived late.

THOUGH: joins the two clauses, ergo it is a subordinating conjunction.

There is a whole list of them but only two (I think) can also be a preposition: before and after.

We left after the movie was finished. We left before the movie was finished. Compare that to:

The car was parked before the murder. [preposition, not a clause] The car was parked after the murder. [preposition, not a clause]

A clause is at the minimum a Subject and a Predicate (It can stand on its own as a full sentence).

Here is a list of the most used ones: http://web.cn.edu/Kwheeler/grammar_subordinate.html