Is "when" used as a preposition?
“When” is not a preposition. As a result, “when” cannot be used as a preposition. Actually, “when” is an adverbial conjunction which introduces or initiates an adverbial clause of time. It is also an interrogative adverb of time. Additionally, “when” can be used as a “hidden “relative pronoun heading an adjectival clause. As it is, in order to get a clear perspective of the two different usages of “when,” pay careful attention to the following illustrative examples.
“WHEN” AS AN ADVERBIAL CONJUNCTION.
EXAMPLES:
When I went to work yesterday, I met an old friend. Note here that whereas the subordinate clause “when I went to work yesterday” is an adverbial clause of “time,” the main clause is “I met an old friend.” Together these two clauses form a complex sentence. In fact, a complex sentence is made up of, at least, two clauses: an independent or main clause and a subordinate or dependent clause.
When I came home from work, I went straight to bed. Note here that two clauses of this complex sentence are: “when I came home from work” (adverbial clause of time) and “I went straight to bed” (main or independent clause).
“WHEN” USED AS AN INTERROGATIVE ADVERB.
When did you return from your trip? Note here that “when” means “at what time.”
When did your brother complete his college education? Note here that “when” means “at what time.”
“WHEN” USED AS A RELATIVE PRONOUN. (In which).
I often reflect on the time when I was a student. Note here that the main clause is “I often reflect on the time” and the subordinate (adjectival) clause is “when I was in school.” Together, these two clauses form a complex sentence. Note here, again, that the adjectival clause “when I was a student” modifies the noun “time.” Actually, the point or question here is “which time?” Of course, the time “in which” I was a student.
Moreover, the same premise applies to the adverb where in that “where” also functions as a relative pronoun. Look at the following examples.
“WHERE” USED AS A RELATIVE PRONOUN. (In which).
I vividly remember the house where I lived as a boy. Note here that whereas “I vividly remember the house” is the main of independent clause, “where I lived as a boy” is the subordinate (adjectival ) clause modifying the noun “place.” Actually, the point or question here is “which house.” Of course, the house “in which” I lived.
I vividly remember the park where I used to play. Which park? Of course the park “in which” I used to play. Note here that the adjectival clause “where I used to play” modifies the noun “park.”
In sum, we see that that “when” is used as an adverbial conjunction, an interrogative adverb, and a (hidden) relative pronoun. It is not a preposition.
