For the reversible reaction : 
${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\rightleftharpoons 2{\mathrm{NH}}_3\left(\mathrm{g}\right)+ \mathrm{heat}$
The equilibrium shifts in forward direction :

by decreasing the concentrations of N₂(g) and H₂(g)

by decreasing the Pressure

by increasing pressure and decreasing temperature

by increasing the concentration of NH₃ (g)