While learning the chopper chapter, you might get surprised at first that how can voltage Eo be zero and the current is not zero.
But if you carefully see the figure, you will find the free wheeling diode.
There are two types of diodes connection
Feedback diode - It supplies power from load to source
Free wheeling diode - It just shorts the load
As you can see in figure 8.4, the diode is connected in parallel to load. So when the diode shorts the load, the voltage across diode is zero and so the voltage across load or the Eo is zero.
This point is very important to understand " As the diode is in parallel, and as the source is disconnected from load by the chopper, the diode shorts the load. Because diode shorts the load, the voltage across load is zero".
But you might ask "why current is not zero then?"
Because there is closed circuit for the current to flow. And as the inductor is present, the current decrease slowly.