It is right?
If I could talked him, I would have told him not to come. I knew that the conference would have been very boring.

Question

It is right?
If I could talked him, I would have told him  not to come. I knew that the conference would have been very boring.

Answer

If I could have talked to him, I would have told him not to come. I knew that the conference would be/was going to be  very boring.
:)

Answer

If I could have talked to him, I would have told him not to come. I knew that the conference would be very boring.

Could have, because I am describing something that  could possibly have happened in the past, but did not: third conditional.

Would be, because I am describing the future from the perspective of that past time: second conditional.

Answer

Has the conference already happened?

If I could HAVE talked TO him, I would have told him not to come. I knew that the conference would BE very  boring.