Why is it so hard to play back a video??

I am new to the android platform and I want to play
back a video file on the screen.

The videos I have are no problem in the video app.

I tried all the samples I could find her, but nothing works for me.

I always get the message "can not play this video"

Is there a working code in the internet to simply play a video?

Where do I place the video file for playback?

My working system is a 2.3. Gingerbread tablet and B4A V. 1.9

Thanks Ralf