How to let a PhD student know that their research is not as good as they think it is I am a postdoc and I have been talking to a PhD student about his research, as his interests are closer to mine than to his advisor's. He is generally nice, smart, and hard working. However, I think he has an unrealistic opinion of his own work.

He has written a paper and wants to submit it to the top conference in my field. (I'm in computer science, so we publish in conferences, not journals.) Unfortunately, I don't think his paper has a chance of getting in. His paper would be a long shot even for a second- or third-tier conference. His paper is also dismissive of prior work, which may offend the authors of that prior work (who are likely to be reviewing his paper).

What should I do about this?

It would be helpful to him if someone gave him a reality check and told him to rewrite much of his paper. However, it is not really my responsibility (or place) to do this. (I'm only two years more senior than him.) If he does submit, he will get a reality check from the reviews, but it would be good if he got some more "gentle" or "constructive" feedback.

I have limited experience with advising. Perhaps someone more experienced can tell me what would be appropriate in this situation. I feel like more than a subtle hint is needed.
Let's start with the obvious:


  I'm only two years more senior than him.


You have a PhD and he has not. You have written (many) papers before and he has not. You know your craft. He does not (yet). And he can learn faster by listening to well-meaning advice from more senior and experienced people. The age-difference does not mean anything. And it seems that you care for his well-being (and his success) and that probably makes your advice even more valuable than the advice of a more experienced, yet indifferent person.  

Given that you are qualified to give this advice, you should absolutely tell him the honest truth. Start by: 


  Would you like to hear my true opinion of your paper? You might not
  like it but I think you should hear it. I think it is better to hear
  it from me than some random reviewer.


And then wait for his answer. If the answer is "No, I do not want to hear your opinion and my paper is so much better than most of the crap they publish in SODA, VLDB or..." then let him know the hard way. Nothing beats delusions better than three strong rejects and merciless bashing from the reviewers. You should also probably let his advisor know what you think of the paper and then let the advisor decide on how to proceed.

Otherwise, if the student is genuinely interested in your opinion then take the time to explain not only what is wrong with his paper but how he can make it better. Explain why he should be more diplomatic in his view of previous work. How he can present his experiments better. How to further clarify his contribution in comparison to the previous works, and so-on. But also state that even if he implements all that, his paper might still not have a chance in first-tier conference. 

Of course, my answer is based on the fact that you do not directly supervise this student. If you do, he has no choice but to hear your opinion. And in this case, crap or half-baked papers should not go to prominent conferences. That makes everyone (co-authors, advisors) look bad. Of course rejection is something normal but only when you submit something that actually stands a chance to be accepted.