Create 6 UserControls for your six questions. Similar to the forms, they return a value of whether their answer is correct. Place all 6 user controls on the form. Use the Visible property to keep them hidden. (Hint: Also consider what you can do with the form's Size property. Start it small, resize it to fit the controls when they get shown. Hint: Design it full sized, then shrink it after the question controls are in place.)
Add a timer to the main form. Set that timer's time based on Easy/Hard to a reasonable value, such as 4 minutes/2 minutes for all 6 questions. On a button click, start the timer and set the 6 questions to be visible.
Tally the score when the timer finishes, or when the user clicks a button indicating they are done. You should be able to do that using the single Start button as both Start and Stop if you create a Boolean variable to know which status the game is currently in at the time the button is clicked. (ie, the first click changes the value of the variable so the second click acts differently.) Show the Results Form as in the previous version of the project.
For bonus points, replace the Easy/Hard setting with the TrackBar control, choosing a timer value between 5000 (5 seconds) and 30000 (6 minutes). Use the online documenation at Microsoft.com to learn about the available properties of that control - it's not just about setting minimum and maximum.