We’ve talked about commonly mixed words before. Effect and affect. Ensure and insure. They can be tricky, but they’re not so bad once you get them down.
Two more sets that we’ve been talking about around here lately are lie/lay and who/whom.

We would never lie to you; this is a tough one. But remember this: Lie, you do yourself. Lay, you do to something else. Lay requires a direct object.

I lie down on the couch.
I lay the pillow on the couch.

John lies down in a pile of snow.
Jamie lays the book on the library table.

Grasping this one brings the question of having a direct object back into play. Who serves as a subject. Whom serves as an object. The same applies when you’re deciding between using whoever and whomever.

Who is the governor of Tennessee?
To whom shall I give these cookies?

Lena knows who pulled the fire alarm.
Natalie knows to whom the honor will be awarded.

The use of whom is really taking some grief these days. Some folks see it as pretentious and archaic. I personally don’t like to use it, but when I do, I sure want to be sure to use it correctly.