An infinitive is a verbal form that consists of the word to and the simple form of the verb: to be, to go, to give, to build. Infinitives are often followed by an object: to give directions, to build a house. Together, an infinitive and its object form an infinitive phrase.
Infinitives can be used in a variety of ways. They may be the subjects or objects of verbs or used after to be+ adjective.
To read the directions is important. (infinitive as subject of a verb)
He forgot to read the directions. (infinitive as object of a verb)
It’s important to read the directions. (infinitive after to be + adjective)
Infinitives can be used as adjective phrases after noun phrases. You will often see this in Structure problems after noun phrases containing the word first.
John Glenn was the first American to orbit the Earth.
Infinitives can also be used to show purpose. In other words, they explain why an action takes place (The phrase in order + infinitive also shows purpose). These infinitive phrases often come at the beginning of a sentence, and are set off by commas.
To learn how to dance, he took lessons.
In order to learn how to dance, he took lessons.
You may see Structure items that focus on passive infinitives. A passive infinitive consists of the word to + be + past participle.
Roberta was the first person to be asked to speak at the meeting.
A gerund is a verbal form that ends in -ing: being, going, giving, building. Like infinitives, gerunds are often followed by objects: giving directions, building a house. Together, a gerund and its object form a gerund phrase.
Gerunds are verbal nouns, and are used as other nouns are used. You will generally see gerunds as subjects or objects of verbs or as objects of prepositions. (Note: Infinitives can also be subjects and objects but never objects of prepositions.)
Playing cards is enjoyable. (gerund as subject of a verb)
He enjoys playing cards. (gerund as object of a verb)
He passes the time by playing cards. (gerund as object of a preposition)
∇ Sample Item
 ________________ the eggs of most birds must be kept warm.
(A) Proper development
(B) By properly developing
(C) They develop properly
(D) To develop properly
The only one of these for phrases listed here that can show purpose is choice (D), an infinitive. This expression means in order to develop properly.
In 1959 the political philosopher Hannah Arendt became the first woman ________________ a full professor at Princeton University.
(A) to appoint
(B) was appointed
(C) to be appointed
(D) an appointment as
After a noun phrase such as the first woman an infinitive is used as an adjective phrase. (B) and (D) are not adjective phrases. Because a passive form is needed (Hannah Arendt receives the action; she doesn’t perform the action), choice (A) is not the correct infinitive form. Choice (C), a passive infinitive, is best.
The ear is the organ of hearing, but it also plays a role in ________________ balance.
(A) maintaining
(B) it maintains
(C) to maintain
(D) maintained
A gerund is used correctly after a preposition. Choices (B), (C), and (D) would not be appropriate after a preposition. Therefore the correct answer is (A).