Close, but not quite. You would say "I don't understand THE difference between A and B." Similarly, you could say "I don't understand how A and B are different." Hope this helps!

You would say, "I don't understand the difference between A and B." You could also say, "I don't understand how A and B are different." THE difference is simply recognizing that there is a difference. HOW it is different is being able to explain that difference. There is a difference. What is the difference? I don't know. I don't understand the difference. How is it different? I don't know how it is different. I cannot even explain or understand the difference between A and B! It's a little tricky. ;-) Does it make better sense to you?

Close. It would be “I don't understand the difference between A and B”