The electric field vector acts in the same direction as the force f vector if q is positive and in the opposite direction if q is negative.

how you understand about that ?

Electric field vector is created by an electric force of a charged particle.

Explanation: The direction of the field vector can be determined by applying Coulomb’s Law which explains the electric force between charge particles. If q is positive the force is repulsive, to a test charge ( which is positive always ) and if q is negative the force is attractive to a test charge.