Actually, your girlfriend can ask the teacher. Ask if you don't understand. If you don't ask, you will never understand. Don't be afraid of the teacher laughing. I don't understand, which means that you have contacted it and have a little impression. Thank you for your practice. You can deepen your understanding. Read the examples and concepts in the textbook repeatedly, and you must understand them thoroughly. Long-term use, will certainly reach the point of perfection.