Real or only imagined?
In a relative clause, the mood tells you whether the thing referred to counts as real and known. The indicative describes something that exists. The congiuntivo describes something that doesn't exist, is still being looked for, or exists only in the speaker's judgement. English makes no such distinction here; Italian does.
Does the person exist?
Indicative: the person exists
Conosco qualcuno che parla cinese.
I know someone who speaks Chinese.
Congiuntivo: the person doesn't exist
Non conosco nessuno che parli cinese.
I don't know anyone who speaks Chinese.
After denied existence
After nessuno, niente, non c'è or non esiste, the relative clause takes the congiuntivo. You'll hear the indicative in casual conversation, but in careful language it comes across as sloppy.
Non c'è nessuno che lo sappia.
There's nobody who knows.
Non ho sentito niente che mi convinca.
I haven't heard anything that convinces me.
Non esiste un metodo che funzioni per tutti.
There's no method that works for everyone.
After superlatives, unico, primo, ultimo
After a superlative or after l'unico, il primo, l'ultimo, the relative clause expresses a personal judgement. Here the congiuntivo is the polished choice and often sounds cautious, like "as far as I can tell". The indicative is common in conversation and presents the judgement as a fact.
Same statement, two registers
Congiuntivo: polished, with reservation
È il film più bello che io abbia mai visto.
It's the most beautiful film I've ever seen.
Sei l'unica persona che mi capisca davvero.
You're the only person who really understands me.
Indicative: conversation, fact
È il film più bello che ho mai visto.
It's the most beautiful film I've ever seen.
Sei l'unica persona che mi capisce davvero.
You're the only person who really understands me.
When it's a plain sequence with no judgement, use the indicative: Il primo treno che parte domani è alle sei.
In the congiuntivo presente the three singular forms are identical: io, tu, lui abbia. That's why the subject pronoun is often added: che io abbia.
In the past
If the main clause is in the past, the congiuntivo imperfetto or trapassato follows.
Non c'era nessuno che potesse aiutarci.
There was nobody who could help us.
Era la persona più generosa che avessi mai conosciuto.
She was the most generous person I had ever met.
