Source code from apk file error on opening

Hi,
I got an apk file.
I extracted the source code from it by the methods provided on the net.
On extracting and importing into the adt it does not shows errors but when i upload the same app on the phone it crashes "unfortunatly the app stopped working"
I am a newbie and i want to learn the code and so if i get the correct code i will be learn it faster..
Is there any reason why the app doesnt start.
Is it because we cannot extract the code properly from the app??
Thanks..