op-amp integrator

The op-amp will modify its output so that the voltage at its 2 inputs are the same. In our case, the op amp must make sure the inverting input is at virtual ground since the non-inverting input is grounded.
This means the op amp output voltage will be such that the current in the input resistor and capacitor is constant.
For that to happen, the voltage across the capacitor must rise (linearly), which means the voltage at the output must decrease (linearly).