What sins does Allah (SWT) forgives and what sins he doesn't forgive?
What if for the sin we have repented deeply and promises Allah to not repeat the sin again. Can you mention in detail

There are no sins that Allah doesn’t forgive if one repents and makes amends.
But if one dies that is a different story, Allah doesn’t not forgive ‘shirk’ – associating others with Allah – other than that, Allah may forgive.
But I wouldn’t take my chances, I would repent and make amends now.
If the sin is against others, I need to get forgiveness from the other parties I have oppressed.