This is the kind of things I know how to use in practice, but would not manage to explain to someone in theory.
Некогда, нечего, нечем, не с кем... they differ from никогда, ничего, ничем, ни с кем in usage.
- Мне некогда - I have no time (literally "to me no when").
- Нечего делать - There's nothing that can be done.
- Не с кем говорить - There's no one to talk to.
- Нечем дышать! - There's no way to breathe!
- Не о чём жалеть - There's nothing to regret.
But:
- Никогда не будет времени - There will never be [enough] time.
- Ничего не делал - I didn't anything.
- Не говорю ни с кем - I don't talk to anyone.
- Не могу дышать ничем! - I can't breathe in any way!
- Я стараюсь ни о чём не жалеть - I try not to regret anything.
So far so good. In practice, I know when to use не- and ни-. But how can the difference in usage be explained, say, in a "scientific" way?
Maybe не- words are used exclusively in impersonal sentences (only with the dative in order to refer to a passive subject, like in "мне некогда"), whereas ни- words are personal, referring to an active subject?
On a side note, which syntax is better?
- Мне не с кем говорить
or
- У меня не с кем говорить