I can remember the english class where the claim was:
When building indirect speech in past, all the tenses are moved towards the past.
He said he was ill = He said: "I am ill"
She told me she had never been to France = She told me: "I have never been to France"
I promised I would do that in five weeks.
How can I express the case where the claim comunicated in the past was and still is valid. Something like:
The teacher said the Sun _ a star.
You're talking about backshifting, which is common in indirect speech, but not required. For events that happened in the past and are complete, it makes more sense to backshift.
My sister said that she wanted to see the movie.
She wanted to see the movie, you all saw the movie, done. End of story.
But let's say you have not yet gone to see the movie. In that case, it makes more sense not to backshift.
My sister said that she wants to see the movie.
This makes it clear that it's still her current desire. But let's say she did want to see the movie, but now she does not:
My sister said that she wanted to see the movie (but she changed her mind, so we'll go without her).
For general statements of truth, it would be most common to use the present tense to indicate a current condition.
My teacher said the sun is a star
However, it's not wrong to backshift. In this case, it's understood that your focus is on the statement, not the general truth. For example, suppose you are talking about an opinion, which is still true, but with which you disagree:
My teacher said that Isaac Newton was the greatest natural scientist of all time, but I think it's Nikola Tesla.
Even though your teacher's opinion has not changed, you emphasize that you disagree with the past statement.
That being said, you can use the present tense to focus on the general truth:
My teacher said that Isaac Newton is the greatest natural scientist, but I told her I would go with Nikola Tesla.
Lastly, you can use the present tense even for past statements that contain general truth. This is like saying that, if you asked the person right now, they'd say the same thing:
My teacher says that the sun is a star.
