Paired conjunctions come in two parts. They connect two equal grammatical ideas, so the words after each part must have the same form. In other words, the sentence must be parallel or balanced.
either ... or
neither ... nor
not only ... but also
*Paired conjunction.
Match adjective with adjective
The new course is both challenging and makes me get bored.
The new course is both challenging and boring.
*Adjective.
*Verb phrase.
*Paired conjunction.
In the incorrect sentence, challenging is an adjective, but makes me bored is a verb phrase. The corrected sentence joins two adjectives.
Match infinitive with infinitive
The professor asked us not only to read the article but also writing a review.
The professor asked us not only to read the article but also to write a review.
*Infinitive: to + base verb.
*Gerund: verb + ing.
*Paired conjunction.
Match noun with noun
The scholarship is available to either undergraduate students or graduate students.
The experiment showed neither a clear cause nor a simple solution.
*Noun phrase.
*Paired conjunction.
TOEFL strategy
Find the first part of the pair—for example, either or not only—and then find its partner. Check the two elements it connects: they should be the same type of grammar, such as noun + noun, adjective + adjective, or infinitive + infinitive.