In vs. IntoHow to use "in" and "into" correctly
To indicate movement from one place to another, use into instead of in:
walked in the room = walked within or inside the room
walked into the room = entered the room
fell in the water
fell into the water
In informal English, this rule is often ignored, and in many contexts it is not necessary to clarify one’s meaning by using into instead of in. In fact, it can sometimes sound awkward to use into in place of the more natural in.
put the receipt into the bag
put the receipt in the bag = more natural-sounding
However, be sure to follow this rule when you need to ensure the clarity of your meaning or are writing in a formal context.
Common Errors in English Usage: Errors in diction and idiom commonly made by native speakers of English
List of Common Errors in English Usage (PDF): Printable version of the complete list
Common Grammar Errors: A list of common errors in grammar (topics like subject-verb agreement and parallelism) as distinct from usage