Rotate the cube on its y axis
The last operation left only the y axis enabled so the Rotate tool should now work only on the y axis. Make sure the Control > Rotate tool is still the currently active tool. Click and drag as shown and notice how the cube rotates now.

Scale cube on its y axis
Select the Control > Scale tool. Click and drag in the Front view as shown to scale the cube. Notice how scaling also is constrained to the y axis.