Helpful for people, useful for objects is a rough rule.

My friend is always very helpful.

When on holiday, a phrase book is useful.

  Ruben, the distinction between "helpful"  and "useful"   is a very fine, or narrow distinction.

 USE refers to the Utility of a thing.  It does a job.  Tools are used to do a job. They are adequate, but not more than merely adequate.  Things which serve a "use" are satisfactory. 

An answer to a question can be "useful" but not necessarily "helpful".  I can give an answer that is merely useful, but not really "helpful".

    For example, you can ask a question  about which   Accent is best regarding the study of English.

You might ask if you should study a  British or American accent.

   My "useful" answer to you would be like this:  "It does not really matter. Study both".

   My "helpful" answer would offer more information.  I could tell you for example, that many foreign students of English operate with a massive misundertanding about the study of English, because they

are convinced that Accent matters; but I could explain to you that in the United States, nobody cares about an accent.  Accents are everywhere in America.

    Another example:  You could ask if the Rules of Grammar are the most important knowledge for you to know to become a confident speaker of English. 

  My "useful" answer  would be to write;  "No.".

 My "helpful" answer  would tell you that  repetitious   pronunciation  exercises  which you   recite aloud every day   will make you a more effective speaker  than will the knowledge of English Grammar.

.

Thanks for your examples Bruce!

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Now, I understand it.