I'm not sure what line to type next?

The test program always keeps the top line in the test text window as the current one which you should be typing.

When you complete a line (successfully), the test program scrolls the test text window up one line, so the line you just typed disappears from the test text window.

You should only need to look at the top line in the test text window, the program will keep moving the test text up so the top line contains the line you should be typing.