Hi,
I got the following issue when I do nested lists.
Basically the line has like a tab before the text and that made a horrible effect.
Any idea how to fix it?
It’s not “like a tab”, it is a tab 
By default numbering and paragraph alignment on lists are done with the use of tabs. If you open the list configuration (last icon to the right on the list toolbar) you’ll see under “Position” how the number and the text is aligned. The problem here is that when the number gets wider than the tab, the text “jumps” to the right creating the gap. You need to either make the tab wider to accommodate big numbers or change the number alignment.
I think the Italian screenshot will not be a problem for you 
(BTW, that’s part of my book on Writer…)
@RGB-es I do not have problem with Italian but I do work in English 
— I do understand why a tabs is inserted seems randomly, but it is a bit tricky compared with Word 2016