thank you for your answer. It is very important for a non-native speaker to catch these subtleties to which a native speaker reacts unconsciously. Let me check whether I understood:
"I need not to know the truth" means that I want to ignore the truth (maybe because it could hurt me).
A doubt: "I need not know the truth" or "I don't need to known the truth" means that it not necessary for me to know the truth, but, if I would know it, it wouldn't make any difference.
In this case, when speaking, should one stress "need"?
"I need". What do I "need"? "not to know the truth".
If one stresses "not" would the same meaning be transmitted? "I need not" What do I "need not"? "to know the truth".
Is it right?