Case of Pronoun [duplicate]

I want to know ---- you talked to. (who or whom?)
I want to know --- the culprit is. (who or whom?)

The simple answer is:

• Always use who. Never use whom.

I.e, who works, and is natural and grammatical in both of the example sentences.

• I want to know who you talked to.
• I want to know who the culprit is.

This is what most native English speakers would say.

However, if you are bound and determined to use whom,
whether you understand the rule for using it or not,
(and if you did understand the rule, you wouldn't have asked the question)
then you should know that

whom can be used in the first sentence

• I want to know whom you talked to.

but whom can't be used in the second sentence

• *I want to know whom the culprit is.
  (that's what the asterisk in front of the sentence above means -- it's ungrammatical).

An easy rule to remember is: Use 'who' whenever you would use 'I/he/she' (subject) and use 'whom' when you would use 'me/him/her' (object):

I want to know whom you talked to
You talked to me

And conversely:

I want to know who the culprit is
He is the culprit

As mentioned numerous times in this forum, the use of 'whom' is gradually declining. But it's always good to know the right way.