How can nucleophilic addition of a primary amine give an imine ?

##R’-NH_2 + (O=)CR_2rarrR’-N=CR_2 + H_2O##
The unsaturated carbon centre of the ketone is an electrophile, whereas the primary amine is a potent nucleophile. Bond formation between nucleophilic and electrophilic sites is entirely reasonable. Once this initial bond is formed, proton transfer can occur so that water may be eliminated (and water is a stable entity, so that its elimination is again entirely reasonable on a thermodynamic basis). I urge you to look at an organic text so that you can see the postulated intermediates.
Formation of imines is normally carried out under Dean-Stark conditions, in conditions of reflux in toluene or benzene, so that water is removed from the reaction gradually. When you do this on a large scale, sometimes you can measure the extent of the reaction on the basis of the volume of water collected in the Dean-Stark trap.