Because all though he knows his sister is dead, he can still hear knocking and scratching sounds coming from her coffin. She's dead, but comes back to life to haunt him causing his madness to grow stronger. 
 
        
             
        
        
        
In a way i would prefer a question because how can you get the answer without given the question or visa versa. So if it would be question the explanation would be it helps you decides who,when,what,where........
And if it would be answer it would be answer helps you can elaborate to help you know more. hope this helps. i tried my best
        
                    
             
        
        
        
It is the set of rules, principles, and processes that govern the structure of sentences in a given language, specifically word order and punctuation. The term syntax<span> is also used to refer to the study of such principles and processes.</span>
        
                    
             
        
        
        
They happen infrequently during the full moon