Thank you very much.
I hope that there are no other cases of using present tense in subordinate clauses when using past tense in main clauses, exept in the case of universal truth and the case of which you've told me.
I just wonder why the things you've just explained me are not written anywhere. At least I couldn't have found them.
"In Indirect Speech [...] the past tense in the reporting verb tends to make the verb of the subordinate clause past tense as well. This phenomenom, known as backshift, is normally optional [...]" (my emphasis added).
Leech (2004.108) writes, of avoiding backshifting:
"The implication of this avoidance of backshifting is that the time of the original utterance ('then') and the time of the of report ('now') are both included within the time-span during which the statement in the reported clause remains valid."