A small child drags a toy car on a string. How is he able to pull the toy car in the horizontal direction?

The applied force on the string is at an angle to the toy car. It can be resolved in two components – a horizontal component and a vertical component. The horizontal component of the applied force is responsible for the motion of the toy car in the horizontal direction.

We see this scenario in any transit station. Passengers routinely pull or drag their suitcases with the help of a strap. The passenger's hand applies force on the strap. The strap is at an angle with respect to the suitcase. The force can be resolved into a horizontally applied force and a vertically applied force. The horizontally applied force can drag the heavy suitcase in the horizontal direction.