In the following sentence, should "Who" or "Whom" be used in the blank space?: ___ understands the contents of this document?
Who understands the contents of this document?
She understands the contents of this document.
If the sentence can be rephrased with he/she replacing the interrogative (who or whom), then the answer is who. If it can be rephrased with him/her, then the answer is whom.
___ is going to water the plants? -> He is going to water the plants. (The correct word is who.)
To ___ am I addressing this letter? -> I am addressing this letter to her. (The correct word is whom.)
It depends on whether you are referring to the subject of the sentence (the noun that is performing an action) or the object (the noun that is having an action performed on them).
If someone is understanding the contents of a document they are the subject, so the correct form is ""Who".
You could use "Whom" if you said "This document was given to whom?"