When you press the jump button, the character suddenly reappears in mid-air and moves (doesn't even accelerate) down. But it should start going up fast when you press the button, with the upward speed slowing, and then start accelerating down.

I still don't understand the problem. Just make jumpStrength greater!

http://www.greenfoot.org/doc/videos.html Choose the video titled "Platform games: running, falling and jumping" That's where I learned about it!

Good progress! And I think that for the text a fixed width font would probably be better, the letter spacing is all wrong for me. (but still readable, though)