We present a method for teaching pick-and-place operation commands, generated from human assembly operation in a teaching environment, that allows a robot with mechanical constraints to perform an assembly task. The "teaching by showing" method, which is based on observing the operations of human workers, has recently been proposed for automatically generating robot commands. This method allows an operator with no knowledge of robots to teach movements to a robot. It is possible, however, that the difference between the degree of freedom of the human operator and of the robot, or differences between the teaching environment layout and the task environment layout may prevent the robot from completing the operation according to the taught sequence. In assembly tasks, the command execution order affects the completion of the assembly task. The method presented here represents operation priorities in the form of a graph, which is automatically generated from recognized human movement operations. When some operation cannot be performed by the robot, the operation sequence is reordered to complete the assembly task, on the condition that the graph remains the same. An example of a robot system is given and the results of a block assembly task are discussed.