Answer:
放学后，他去图书馆看书。Without the comma, it would be 他放学后去图书馆看书。
Explanation:
The literal translation of the first one: After school, he goes to the library to read books. The second one is: He after school goes to the library to read books. 放学=after school 后=after 他=he 去=go/goes 图书馆=library 看=see (in this case "read") 书=books
