What they can't do

What sleep can't do:
Do your requirements while you're in dreamland.

What a pessimistic mind can't do:
Anything---

What a worrywart can't do:
Use time wisely.

What a preoccupied person can't do:
Think of beautiful things.

What an overload of schedule can't do:
Adjust its time for you.

What stress can't do:
Kill its killer.

What seemingly endless tasks can't do:
Endure forever.

What you can't do:
He can do.