I wouldn't worry about it, for a few reasons. Your reactions to the piece are your reactions to the piece, as a reader. That's valuable. You can say that it's not your preference, or that it doesn't work, or whatever, but the point is you, as a reader, had an issue with something. You can suggest, in your opinion, why it might be a problem and something the author could do to fix it. It's up to the author to determine if she or he agrees and wants to follow your suggestion, because it's just that--a suggestion. Everything you post should have an implied "IMO" attached. We're not professionals (save for the few here who are), and we're all just trying to help each other. There's no reason anyone should be treating anything any of us says as gospel, and if you do, well, that's really your own fault. Secondly, and I know the other members of the old guard here will agree, writing a critique is more a benefit to the critiquer rather than the author. It's a way of honing your editing skills and giving you insight into why you think certain things don't work and how you think they should be fixed. You can then apply it to your own writing. There's no guarantee that your critique will be useful to the author. But there's a 100% guarantee it'll be useful to you. Whatever you do, don't avoid critiquing because you're worried about not being good at it or misleading the author. If it makes you feel better, deliberately state that what you're suggesting is your opinion and that it may or may not work for the author. But I'd hope the author understands that this forum is really a group of fellow writing students who are just offering their opinions anyway.
