Past Perfect Continuous vs. Past Continuous

Understanding the difference between these two tenses is rather difficult for ESL students. Look at these two sentences:

• She was studying for 45 minutes before I interrupted her.
• She had been studying when I interrupted her.

Both of them are incorrect. The Past Continuous has been mixed up with the Past Perfect Continous here.

• She had been studying for 45 minutes before I interrupted her.
• She was studying when I interrupted her.

The first sentence requires the Past Perfect Continuous because the action of studying began one time earlier. The other sentence requires the Past Continuous because "studying" is just an interrupted action in progress.