Dear Sam,
I am very glad you can invite me to your home next weekend. I would like to go but I’m afraid I can’t. First of all, I have a test next Monday, so I have to study for the test. The test is very important because it makes up 30% of the final exam. I must do well in it. Also, I have to finish my homework. What’s more, I have to stay at home helping my mother do some housework. I wish I can go next time.
Bill