How to use who or whom in sentences? How would I know when to use the word who or whom in the following? Also,is the who whom a subject,...

The selection of who or
whom, whoever or whomever is
dependent upon the case that is needed in the sentence or clause in which the relative
pronoun is used.

• The
  nominative case is used whenever the pronoun
  who is the subject of the sentence or clause, or the predicate
  nominative, a pronoun that follows a linking verb such as to
  be. Here are some examples:

Miss
Skiffins is the person who is in
charge of casting for the play.
[subject]

Whoever studies
ancient history knows about the polytheism of the Greeks and Romans.
[subject]

Miss Skiffins is who
I am. [predicate nominative]

• The objective case
  whom or whomever is used when
  the pronoun is a direct object, indirect object, or object of a
  preposition.

One of the wise men about
whom we wrote was Solon. [object of the preposition
about]

You may give
whomever you choose this treat. [indirect object (to) is understood
before whomever]

Such people as the polytheists had several
or even many gods whom they worshipped. [direct object -
they worshipped whom]

**Hint:  In order
to determine which case to use, find the verb that is the predicate of the clause in
which the relative pronoun acts and determine what is the subject.  If there is another
noun or pronoun in the clause, many times it is the subject (especially with active
verbs), so you need objective case for the relative
pronoun.

For instance, in your example Guess
(who/whom) I met this morning, say the verb met and
ask "who met?"  The answer is the pronoun I, so the
position of subject goes to I. Therefore, use
whom.  ( I met whom = the direct
object)

Likewise, in the next example,
whom is used for the same reason:   As an orator he is a
man (who, whom) no one can accuse of using a few words
where many will do.  No one is the subject of the clause,
so the only position open is direct object. (no one can accuse whom = direct
object)

In addition, the first two sentences use the
objective case whomever you can trust [direct
object] and whom you were thinking of [object of
preposition of ]  for the same reasons given
above.

However, the last example has no other noun or
pronoun that can act as the subject so who is the correct
choice:

Library privileges will be withheld from those
who have been found guilty of mutilating borrowed books.
(____have been found guilty -- a subject is needed)

So
often we are confused about these relative pronouns because people do not always say the
appropriate one and we learn through listening the ungrammatical use.  So,
since converstional English is more lenient in its rules, we must go through the little
"test" of finding the subject of the verb in the relative clause in order to choose the
appropriate relative pronoun.