To me: "Useful" is more functional, necessary, practical and without emotion. You often describe THINGS are useful, even if they are non-physical. Useful advice = to get something done, achieve something. Examples: - useful travel tips e.g. make sure you get to the Eiffel Tower early, before the crowds arrive. - useful recommendation e.g. that Joe is a good plumber in your area - useful instructions e.g. those that come with IKEA furniture (well, maybe...) - useful advice e.g. shortcut to avoid the traffic jam on the motorway - useful tip for learning English e.g. try listening to podcasts - useful idea e.g. ingenious way to use old plastic bottles to power a car! - useful knowledge e.g. about first aid or how to fix a car Plus some idiomatic uses, examples: - "why don't you make yourself useful, and lay the table?" (but I would still never say that a person is useful) "Helpful" is more emotional, nice and kind. It is often used for PEOPLE and the THINGS PEOPLE DO. Helpful advice = makes something easier or lighter; it is supportive and nice (not such a yes/no achievement). Maybe I feel grateful and glad. Examples: - helpful memory e.g. it was helpful that I remembered the way, otherwise I would have been lost - helpful support e.g. Sarah did my shopping while I had a broken leg, it was so helpful! - The exam was very difficult, so it was helpful that I had did so many practice exams in advance - If your child has trouble sleeping, it is helpful to keep a bedtime routine - My doctor gave me some helpful advice to lose weight - It was so helpful that I could speak English during my holiday in Australia - There's a helpful mnemonic to spell "rhythm": Rhythm Has Your Two Hips Moving In practice, the definitions are not sooooo different, much advice can be useful and helpful. I am just trying to give you examples to get a feel.

This is definitely some fine hair splitting and you'd be totally fine just using them interchangeably. The closest I can come to a nuance would be saying "useful" feels more "utilitarian" to me. You would never call a person "useful" because you would risk seeming somewhat like a sociopath, regarding people as objects. But a dictionary can definitely be both useful and/or helpful and in that case the words are pretty much identical. A lot of times the issue with nuances in synonyms like these is that they don't quite exist in the words themselves, they exist in what are called "collocations," that is to say, set phrases or pairs of words that co-occur quite often. In the case of these two words there is a surprising amount of collocation overlap, but one I can say pretty confident is mostly used with "useful" is "become": something will "become useful" but we don't often say it will "become helpful". I initially started this explanation out with a bit of a rambling paragraph about a sense of time with regard to these two words. I believe that the "become useful" collocation can illuminate it a bit more: the word "useful" has significantly more sense of a habitual future. We often say an object could "prove useful" and that is why we don't want to throw it out. We don't often say helpful in this context. But honestly, to be able to recognize the nuances in synonyms like these it really just takes listening to and reading a lot of English. And no one is going to fault you for saying "I don't want to throw out that broken blender, it could become helpful," except insofar as to call you a hoarder. ;-)

Try writing some sample sentences and then post them for feedback.

How do you describe something useful but not helpful? Because I know that sometimes something can be useful but not helpful.

Something that you can use is "useful" to you, and something that helps you is "helpful" to you. So it would be "a helpful explanation/advice", and "useful knowledge".