The ortho form is more stable than para form. At room temperature about 75% of ortho hydrogen is present. At low temperature, there will be a greater proportion of para hydrogen. It's possible to determine their proportion because the two forms differ in certain physical properties like specific heat, thermal conductivity etc. On heating above 800°C, para hydrogen fully converts into ortho hydrogen. When atomic hydrogen is made collided with para hydrogen, ortho hydrogen is formed.
Nascent hydrogen is produced in the reaction mixture in solution in the presence of oxidizing agents. But atomic hydrogen is produced at a high temperature of about 3000°C by dissociating H2 molecule. It can't be produced in solution at ordinary temperature. Nascent hydrogen can never be isolated but atomic hydrogen can be. Atomic hydrogen is much stronger reducing agent than nascent hydrogen. For example, it can reduce certain metallic compounds AgAl, CuO, CdS to the respective metal while nascent hydrogen can not.