Coordinate conjunctions join words, phrases, or clauses. The most common ones in TOEFL grammar questions are and, but, and or. When a conjunction joins two parts of a sentence, those parts should have the same grammatical form. This is called parallel structure.
I like to read and writing.
I like to read and to write.
I like reading and writing.
*Infinitive: to + base verb.
*Gerund: verb + ing.
*Coordinate conjunction.
The verb like can be followed by either an infinitive or a gerund. However, when and connects two activities, both activities need the same form. Do not join an infinitive with a gerund.
Adjectives joined with coordinate conjunctions
The car is not fast but efficient.
She may be angry or sad.
*Adjective.
*Coordinate conjunction.
Each conjunction joins two adjectives. The sentence remains balanced because both sides describe the same noun.
Nouns joined with coordinate conjunctions
She is not a nurse but a doctor.
We need to talk to the coordinator or to the assistant.
*Noun or noun phrase.
*Coordinate conjunction.
Verbs joined with coordinate conjunctions
He does not clean but cooks for her.
You can run or walk during the race.
*Verb.
*Coordinate conjunction.
Two clauses joined with coordinate conjunctions
A clause has its own subject and verb. When and, but, or or joins two independent clauses, use a comma before the conjunction.
The researcher collected the data, and she analyzed it carefully.
The lecture was long, but the students remained interested.
You can submit the form today, or you can submit it tomorrow.
*Subject.
*Verb.
*Coordinate conjunction.
Each side of the conjunction can stand alone as a complete sentence:
She analyzed it carefully.
Do not use a comma if the two verbs share the same subject.
The researcher collected the data, and analyzed it carefully.
The researcher collected the data and analyzed it carefully.
In the correct sentence, the researcher is the subject of both verbs. The second part is not an independent clause, so no comma is needed.
Phrases joined with coordinate conjunctions
Conjunctions can join longer units too. The two phrases should still match.
The students were asked to complete the survey and submitting it online.
The students were asked to complete the survey and to submit it online.
*Infinitive phrase.
*Gerund phrase.
*Coordinate conjunction.
TOEFL strategy
When you see and, but, or or, look at the grammar immediately before and after the conjunction. The two parts should match: adjective + adjective, noun + noun, verb + verb, clause + clause, or phrase + phrase.