We add -ne
1. when asking a yes-or-no question, that is to say, when we ask if a certain assertion is true or false, e.g.
Videsne canem? – Do you see the dog?
This is when we are asking neutrally. If we want to signal we are expecting or hoping for a negative answer, we can use the word num:
Num canem tetigisti? – Did you really touch the dog? / Surely you did not touch the dog? etc.
Or, if we want to signal we are expecting a positive answer, we can use nonne (which is actually non + -ne):
Nonne canem vidisti? – Did you not see the dog?
Simple -ne is neutral.
2. when asking for an alternative between two choices, where the “or” is always an:
Uter nostrum popularis est, tune an ego? – Which one of us two is a man of the people, you or me?
You can also leave the -ne out in this case (tu an ego) or use the word utrum:
Utrum ea vestra an nostra culpa est? – Is it your fault or ours?
Plot twist: To ask emphatically, you can also say utrumne, and an can be emphasized by saying anne.
By the way, all I just told you does not just apply to direct questions, but also to indirect questions, e.g.
Honestumne factu sit an turpe, dubitant. - They are unsure whether it is an honourable or shameful thing to do.
We do not add -ne when we have a question word like quid or ubi.