Why is the resistor in parallel with the voltage source open and the resistor in series with the current source short? How can you tell by the circuit diagram?

That half-assed teacher told you that? Your question itself is a ridiculous wrong question! When a resistor is connected in parallel with a voltage source, a current will flow. Although the output voltage of the voltage source remains unchanged, the resistor is consuming power. How can this be a way out? Don't chew. That resistance will be infinite! Similarly, the resistor is connected in series with the current source, and there must be a current flowing through the resistor to produce a voltage drop. How can it be called short circuit? Don't tell me the resistance is zero. Understand the basic terminology of electrotechnics first, and then say: "Open circuit must be that there is no current flowing in the circuit and no power consumption". "Short circuit means that the resistance between two points is zero, that is to say, if there is current passing through, there is no voltage drop between two points of short circuit!"

So your question itself is a funny wrong question! That statement is unscientific! The original intention of estimation is that when the voltage source and a resistor are connected in parallel to form a whole to supply power to the external circuit, the original resistance can be ignored when calculating the voltage, current and load data of the external circuit. Similarly, when the current source and the resistor are connected in series to form a whole to supply power to the external circuit, the original resistance can be ignored when calculating various data of the external circuit.