If you can’t, you must. If you must, you can.
如果你不能,你必须。如果你必须,你就能。
I must confess that I never could see any beauty in her. Her face is too thin; her complexion has no brilliancy; and her features are not at all handsome. Her nose wants character; there is nothing marked in its lines. Her teeth are tolerable, but not out of the common way; and as for her eyes, which have sometimes been called so fine, I could never see anything extraordinary in them.
"Books can be mirrors, windows, and sliding glass doors."