Prepositions and Conjunctions

You might have come across the terms 'Prepositions' & 'Conjunctions' while reviewing SC problems and while you don't really need to know them in detail, here's a basic explanations of the two.

Even though inherently both Prepositions as well as Conjunctions are connectors, Prepositions are used to connect a noun element to a sentence whereas Conjunctions have the ability to connect two verbs together which means they can actually connect two sentences to each other.

Examples of Prepositions - on, over, to, from, about, for, against, with, between, but, etc.

Examples of Conjunctions - and, nor, but, or, then, for, since, etc.

As you might have noticed words such as 'for' and 'but' can act as both Preposition as well as Conjunction; then how do you figure out when it's being used as what?

Try this rule - divide the sentence into two parts (1 part before the preposition/conjunction and the other part after that). Now if the two parts make sense on their own then they are two different sentences and we can only use conjunctions to join two sentences so the word in questions is a Conjunction, else it is a Preposition.

Consider the following examples:

1. I have been living in New York since last year.
   
   
2. I have been living in New York since I passed my exams.

Now in the sentences above, the word in question is 'since'.

If you break up sentence 1 across 'since', the two parts are 'I have been living in New York' and 'last year'. Now while the first part 'I have been living in New York' makes sense the second part 'last year' makes no sense. Thus 'since' is acting as a preposition here connecting the noun 'last year' to the rest of the sentence.

Now doing the same thing with sentence 2 we get 'I have been living in New York' and 'I passed my exams', both of which make perfect sense on their own. Thus 'since' is acting as a conjunction here joining two sentences.

To Sum it Up

1. Prepositions are used to connect a noun to a sentence.
   
   
2. Conjunctions are used to connect two verbs or sentences.
   
   
3. The same word can be used as both conjunction as well as preposition, depending on the context.