The word ‘when’ is not a preposition; it is primarily used as a conjunction and an adverb. As a conjunction, it introduces dependent clauses that refer to time.
For example, in the sentence ‘I will call you when I arrive,’ ‘when’ connects two clauses: the main clause (‘I will call you’) and the subordinate clause (‘I arrive’). Here, ‘when’ indicates the time at which the action of the main clause will occur.
Additionally, ‘when’ can function as an adverb, as in the phrase ‘When will you be home?’ In this case, it is asking about the specific time of an event.
In contrast, prepositions are words that show relationships between nouns (or pronouns) and other words in a sentence. Common prepositions include ‘in,’ ‘on,’ ‘at,’ ‘by,’ and ‘with.’ Since ‘when’ does not fit this definition, it cannot be classified as a preposition.