A subordinate or dependent clause is a clause that does not make sense by itself, even though it has a subject and verb.  It needs the independent clause to make sense.  The dependent or subordinate clause can be used as an adjective, an adverb or a noun in a sentence.  In this practice, the dependent or subordinate clause is used as a noun....that is, it will be used in a sentence as a noun can be used, as the subject, direct object, etc.

Examples...

What he wrote was clear.

•  What he wrote - subject

I don't know how he can be reached.

•  how he can be reached - direct object

Give whoever answers the message.

•  whoever answers - indirect object

I sent the notice to whoever was interested.

•  whoever was interested - object of preposition "to"

This is what I intend.

•  what I intend - predicate noun
  
   

Words that introduce a noun clause....

Many words that introduce a noun clause can introduce other kinds of subordinate/dependent clauses.  To tell them apart, determine how they are used in the sentence.

Example

Phil noticed that the phone was buzzing.
Phil noticed what? - that the phone was buzzing - direct object

The phone that was buzzing was off the hook.
"that was buzzing" tells which phone was off the hook - thus, it is an adjective clause.

To decide whether to use "who" or "whom" in a noun clause, decide how the word functions in the noun clause.  As a subject in the noun clause, use "who" or "whoever."  Use "whom" or "whomever" as the object of the verb or the object of the preposition that is within the noun clause.  Look for how the relative pronoun is used in the clause....not how the clause is used in the sentence.  The use of the pronoun tells you whether to use who(ever) or whom(ever).