how nature of nucleophile decides SN₁ or SN₂ ?

Dear jee

 The key difference between SN1 and SN2 is when the nucleophile
participates: in the rate determining step in SN2, but after the rate determining step in SN1.
Thus the course of the reaction can be controlled by the nature of the nucleophile. Thus, other
things being equal, a strong nucleophile favors SN2, while a weak nucleophile favors SN1.
Nucleophilicity depends on basicity, polarizability, and solvation. For the same series, anions
are better nucleophiles than their neutral counterparts ( i.e. OH- is better than H2O). In the
same series in protic solvents, increasing polarizability leads to increasing nucleophilicity,
thus I- is a better nucleophile than F-.