What are the conditions for hydrogen bonding?

These are:
(i) Hydrogen atom should be bonded to highly electronegative atom (N, O, F). The strength of the hydrogen bond increases with an increase in the electronegativity of the other atom. For example, the electronegativity of N, O, F increases as N < O < F.
Therefore, strength of hydrogen bond also increases as,
N – H ... N < O – H ... O < F – H ... F
(ii) The size of the electronegative atom should be small. Smaller the size of the atom, more the electrostatic force of attraction, stronger will be the hydrogen bond.