Could you please tell me the difference between these 3 sentences.
Brian studied chapter 2. Fine. Just tells us a fact about the past. Maybe he did this 20 minutes ago. Maybe he did 20 years ago. Brian has studied chapter 2. He did this in the past, and it has some import
For the third sentence it would be correct to say, "Brian has been studying chapter 2." This is the present perfect progressive tense, emphasizing the continuing nature of a single or repeated action that began in the past and that has continued at least up to the present.