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

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

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.

What is the trouble? Is the character falling through the floor?

It'd be good if the lines could manifest themselves as lines rather than a series of dots - you could achieve this by drawing a line between the previously placed dot and the new one whenever one is drawn.

One other thing I forgot to mention - the man sinks into the ground sometimes when jumping - this is a relatively common bug and shouldn't be too difficult to solve :-)