Hi Pter, Don't be sorry. I could be wrong. As you can see an expert has just disagreed with me :( I'm always wrong and your English is much better than mine. I was just giving my two cent opinion and probably useless.
Yes. I do know for a fact that the sun is one of billions of stairs in the universe which is what my example needed to show doubt.
A is quite certain that he/she is right about the star fact. But someone (maybe someone he/she thinks is credible) says the sun is another planet. A new theory she supposed since nobody has been to the sun.So A says "I thought the sun was a star, composed of helium and other flamable gases..yada yada yada "
Again, I'm just a learner. Dont tkae my word for it.
(Edit: This is probably less related and I believe you already know it. In reported speech, you can say facts in the past tense. My teachertold me that the sun was a star.)