Input resistance of ideal op. amp
Here the opamp is used in a circuit where it will try (and succeed since it is a proper circuit) to keep the voltage between the + and - inputs zero Volts.

The + input is grounded.

The opamp's output can only influence the - input via the 100 kohm feedback resistor.
The opamp will do "whatever is needed" to keep the - input at 0 V

So whatever you do at vi, the other side of that 10 kohm resistor will be at zero volt. This is called "virtual ground" because it is not connected to ground but behaves like it is.

So at vi you see a resistor of 10 kohm to ground. This means the input impedance of this circuit is 10 kohm.

Note how that 10 kohm resistor to ground does actually nothing. You could remove it and nothing changes. Why ? Because there is zero volt across it. Meaning no current flows meaning it is "not there". Note that this is only true when the opamp is ideal, in a practical circuit the 10 kohm to ground does influence the circuit's behavior.