Some conjunctions are used in pairs, they are called correlative conjunctions. They are used to show the relationship between ideas expressed in different parts of a sentence.
Most are coordinating correlatives. These link words and phrases.
both ... and
either ... or
neither ... nor
not only ... but also
He was not only a scoundrel, but also a cruel man.
We use either…or together when we want to link two positives:-
Either the president or the vice-president will go to the conference. (The president will go to the conference, or the vice-president will go, not both of them.)
We use neither…nor together when we want to link two negative ideas:-
Neither the president nor the vice-president was able to solve the problem. (The president couldn't solve the problem and the vice-president couldn't solve it either.)
Some are subordinating correlatives.
if ... (then)
less ... than
more ... than
so ... that
If Barbie is so popular, (then) why do we have to buy her friends and boyfriends?
There is less water in the aquarium than there was last week.
She ate more gummy bears than usual.
She was so hungry that she could have eaten a horse.