Conjunctions, relative pronouns and relative adverbs can be used to connect two clauses. The grammar is different. Prepositions do not connect two clauses. They merely show the relationship between a noun/pronoun and another word in the sentence.
Relative pronouns
Relative pronouns are different from conjunctions. Relative pronouns are important connecting devices. They not only connect two clauses but also act as the subject or object of the verb in the relative clause. This is the main difference between conjunctions and relative pronouns. Conjunctions merely connect two clauses. They do not serve any other purpose.
Study the sentences given below.
This is the letter. My mother sent me this letter.
We can connect these two clauses.
This is the letter that my mother sent me.
Here the relative pronoun that replaces the phrase this letter and acts as the object of the verb sent. It is easy to decide whether a relative pronoun is the subject or the object. When it is the object it will be immediately followed by another noun which acts as the subject. In this case, the relative pronoun ‘that’ is followed by the noun ‘my mother’ which acts as the subject.
Another example is given below.
This is the boy. He won the first prize.
This is the boy who won the first prize.
Here the relative pronoun who replaces the pronoun he. It acts as the subject of the relative clause.
Relative adverb
The relative adverb not only modifies a verb, but also joins the two clauses in a sentence. Note that a relative adverb does not act as the subject or object in the relative clause. It merely replaces an adverb.
This is the house. John lives here.
This is the house where John lives.
Here the relative adverb where replaces the place adverb here.
Another example is given below.
I first met Susie on that day. I will never forget that day.
I will never forget the day when I first met Susie.