Well, there's nothing stopping you but etiquette? Like, if the author has asked for concrit, go nuts (but maybe try to balance negative with positive). That's about the only time it happens on AO3.
If they've not... then you could say that you loved the story but think some of the writing would be even better with a beta or something like that. Or you could ask if they'd be willing to take concrit. If they say no, walk away. And if they say yes, try to take it to a private message space if possible.
And don't be mean about it. People often are and that's not at all helpful.
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(Anonymous) 2015-10-31 02:23 am (UTC)(link)If they've not... then you could say that you loved the story but think some of the writing would be even better with a beta or something like that. Or you could ask if they'd be willing to take concrit. If they say no, walk away. And if they say yes, try to take it to a private message space if possible.
And don't be mean about it. People often are and that's not at all helpful.
Hope my advice helps!