Design mosfet circuits at different corner cases

Hi everybody
I have a problem with design a circuits which it has to work at any corner case but it dosen't work.
How can I design a mosfet that it will be work at any corner cases?
My teacher said that if you want to design a circuits which it work at different corner case you have to look at Vdsat value.
I don't know what is relation between vdsat and vds in order to work properly.
Does anyone can help me?

What I do is, go to the problem case and find the
problem transistor(s) and see what their behavior
problem is and what their main parametric excursion
is. Straight debug, case by case.

Vdsat/Vds applies to only one thing, DC gain as
affected by Rout. But you don't know whether that
Rout is dominated by the gain device or its load
device, necessarily, and you don't know whether it
comes from VT being high, from bias current being
high, from short channel devices (deltaL driven to
its extreme) or even if it's really a gm rather than
Rout problem. But you can dope some things out,
like short channel will degrade Rout but raise gm,
etc.

This is probably the lesson that's trying to be taught.
But there's a million ways to die, not just one.

Thank you for your reply
But unfortunately I didn't understand it
could you make an example please
How do you do that?