What is the grammatical error in the sentence: "After reading the book the students divided into discussion groups."?

The sentence "After reading the book the students divided into discussion groups." contains a grammatical error related to punctuation, specifically the lack of a comma after the introductory phrase. The phrase "After reading the book" serves as an introductory element providing context for the main action of the sentence. In standard English grammar, it is customary to place a comma after such introductory phrases to enhance clarity and indicate a pause before the main clause. This helps the reader better understand the structure of the sentence and the relationship between the introductory information and the action that follows. Therefore, inserting a comma after "book" would correct this error and improve the readability of the sentence.