An object of a preposition is a noun, pronoun, gerund or noun clause that comes after a preposition, such as in, at, of, to, by, behind, on, and so on, to form a prepositional phrase.

(After his exams) Tom will take a trip (by boat).

This sentence contains two objects of prepositions. Exams is the object of the preposition 'after'; and boat is the object of the preposition 'by'.

An object of a preposition can cause confusion in structure questions on the TOEFL test because it can be mistaken for the subject of a sentence.

Example:

With his friend _____ found the movie theater.

(A) has

(B) he

(C) later

(D) when

Answer:

In this example you should look first for the subject and the verb. You should notice the verb 'found' and should also notice that there is no subject. Do not think that fiend is the subject; 'friend' is the object of the preposition with, and one noun cannot be both a subject and an object at the same time. Because a subject is needed in this sentence, answer (B), he, is the best answer. Answers (A), (C), and (D) are not correct because they cannot be subjects.

==========================

A preposition is followed by a gerund or noun clause that is called an object of the preposition. If a word is an object of a preposition, it is not the subject