Explain how does the presence or absence of hydrogen on N atom of amines affect the modes of their reactions with nitrous acid.

The three classes of amines react differently with nitrous acid. Nitrous acid is very unstable and is generated only during the course of a reaction using NaNO₂/HCl.
Primary amines have two hydrogens present on N-atoms.
Aromatic primary amine from diazonium salt with NaNO₂/HCl

While Aliphatic primary amine form diazonium salt which is unstable and decompose to liberate N₂ gas and form alcohol,

Secondary amines contain one hydrogen on N-atom. Both aliphatic and aromatic amines undergo electrophilic substitution reaction with HNO₂ to form nitrosamines.

Tertiary amines have no H present on N-atom. Aliphatic tertiary amine form salt with HNO₂ which is water soluble.
R₃N + H—ONO → R₃NH⊕®ONOΘ
Aromatic tertiary amine undergo electrophilic reaction at para position as the ring is highly activated by NR₂ group.