We know that when alkyl halide reacts with AgNO₂ then it formsnitro alkanebut when it reacts with KNO₂ then it forms alkyl nitrite... how can v knw that when will it join with O and when with N ..........???????

The bond between Ag-O in AgNO₂is covalents, therefore the lone pair of electrons present on Nitrogen is freely available for attack in nucleophillic substitution in alkyl halide resulting in the formation of nitro alkane. Whereas the bond between K-O in KNO₂is ionic in nature and Oxygen attracts the shared electron density towards itself and also since oxygen is more electronegative than Nitrogen, hence the negative charge on oxygen is available for attack in the nucleophilic substitution leading to formation of alkyl nitrite.