Apfelsaft trinke ich vs. ich trinke Apfelsaft?
What's the difference between them, and which one is correct and why?
They are both correct, in German the important thing is to keep the verb at the second place but the subjet and the complement can be swcitch of position. For instance : "Ich habe zwei Schwestern" can also be written like this : " Scwhestern habe ich zwei". You will be understood with the both sentances. Get it ?
It also has a bit difference between those in terms of how to emphasize the answer. The first one is used to tell you are drinking juice at the moment especially when someone ask you what are you drinking. But the latter is like a simple sentence that shows you can drink apple juice. :)