Would in a sentence be grammatically correct?

No, "in" by itself is not a grammatically correct sentence. It needs a subject and a verb to form a complete thought.

Here are some examples of how you could use "in" in a grammatically correct sentence:

* "The cat is in the box." (Subject: cat, verb: is, prepositional phrase: in the box)

* "I am in the car." (Subject: I, verb: am, prepositional phrase: in the car)

* "He walked in the door." (Subject: he, verb: walked, prepositional phrase: in the door)