I put a video my phone and i can only see the top quarter of it. The rest of it is covered in a green solid color. Sound works fine and it works on pc

i'm not expert but looks like something wrong with the encoding of video

I got this with a video too using the standard player.

Instead I installed Mplayer and then the front-end KMPlayer for it.

I think they're both in extras-devel and they work just fine, remember to set Mplayer as the chosen player in KMPlayer.

Happened to me too with some old Star Trek TV recordings.
Don't know what video you tried to play, but for me the cause is the old DiVX3-Codec. Basically that version of the DiVX encoder was at least suspected to contain pirated code (not sure wether they could actually prove it) and was therefore officially discontinued and abandoned for legal reasons. I don't remember all the details, but more or less the entire codebase was rewritten afterwards if I recall correctly

Keeping this in mind it's not all too surprising that modern hardware lacks support for decoding it.
I still find it sad though, Since the top part is decoded flawlessly the "green block decoding error" seems somewhat artificial.
My guess is that the DSP-chip COULD decode the video, but chooses not to for legal reasons.

Oh well, the "non hardware-accellerated" applications can still decode it and for my usage the N900's CPU is fast enough to do it in software, so it's a matter of energy usage at best...

Tried mplayer and the video worked but the sound didnt work with this.