Words that usually precede nouns or pronouns and express their relation with other parts of the sentence are known as prepositions. Prepositions can be of several types, depending upon their usage. The prepositions "in", "on", and "at" are prepositions of time as they are used to refer to a point in time. For example, "in the evening", "at midnight", and "on Wednesday". Thus, "inside" is not a preposition of time; instead, it is a preposition of place.