No, “in” is not always a preposition, but it is most commonly used as one.
“In” as a Preposition
When used as a preposition, “in” shows location, time, or condition.
- Location: The food is in the fridge.
- Time: We will meet in an hour.
- Condition: She is in trouble.
“In” as an Adverb
Sometimes, “in” functions as an adverb, typically indicating movement toward an enclosed space or participation.
- He walked in (movement inside).
- She joined in the discussion (participation).
“In” as an Adjective
In informal usage, “in” can also function as an adjective to mean fashionable or currently popular.
- That color is really in this season.
So, while “in” is most often a preposition, it can also be an adverb or an adjective depending on its usage.
