Thanks,Xray!
Welp,about your suggestions:
1.Oh,sorry! I forgot to turn off the clouds
2.Keys.IsDown is too difficult to make one and it doesn't trigger once when it's pressed,i triggers the code like 300 times per second when it's pressed
3.Haha,you don't need to press "Shift-A" or "Shift-Z" you just need to press "a" to upshift and press "z" to downshift