With the use of consistent orderings in epsilon approximately rho s/a and delta approximately kperpendicular to rho s model equations are derived for the drift instabilities from the electrostatic two-fluid equations. The electrical resistivity eta included in the system allows the dynamics of both the collisional drift wave instability ( eta not=0) and the collisionless ion temperature gradient driven instability ( eta =0). The model equations also include effects of sheared velocity flows in the equilibrium plasma. The model equations used extensively in earlier nonlinear studies are obtained as appropriate limits of the model equations derived in the authors' work. The effects of electron temperature fluctuations are also discussed.