Default Source code from apk file error on opening

Hi, I am a noob
I downloaded an apk file.
I extracted the source code from it by the methods provided on the net.
I want ot learn the coding of the app..
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??

I want to kwon how this app is coded
for that i want to check the source code files