How do I Make my character fall?

This is really basic but I can't seem to be able to do it. I have my character who I need to fall down. My problem is that when I press left or right my character stops falling in mid air and moves to the right or left like he is walking on air. I have put physics on. To make my character go left and right I'm using:

When key is pressed, set speed =200.

Help will be appreciated, thanks very much!

Try using the "platform" behavior instead of physics

Thanks for helping me, it works great. This is so obvious xD Im quite new to all this...