Prepositions are always followed by a noun, not the verb. The noun behaves as an object. It can be either a noun, proper noun, pronoun, noun group/noun phrase, and gerund. Prepositions (as, in, at, of, to, by, behind, on) followed by a noun phrase to form preposisional. List of other commonly used prepositions can you see in the Appendix. Note the following sentence :
(After his exams) Tom will take a trip (by boat).
The above sentence contains two objects of prepositions. "Exams" is the object of a preposition "after". While "boat" is the object of a preposition "by".
An object of the preposition can also give rise to confusion in TOEFL STRUCTURE test questions. So, You can just think of these objects as a subject in a sentence.
Example 1 :
In this example, the first time you should look at is the subject and verb. It appears that "found" as a verb, but not found the existence of the subject. Don't think that "friend" is the subject, but "friend" is the object of the preposition "with". Please note that the one noun can not be subject and object at the same time simultaneously. Because it is needed in the above question is to find a subject, then the answer to (B), he, is the best answer. While (A), (C) and (D) is not the right answer, because the three of them could not be the subject.
STRATEGY 2
The following formula is the formation of a preposition
( [Subject+Verb] + Prepositions + Noun )
A preposition is always followed by a noun (noun, pronoun, gerund), not the verb.