First of all, let me say-- there's no 'wrong' way to write a fic comment (unless you are being rude or offering unsolicited concrit). Short comments are good, emojis are good-- all of it's good! Don't take me making this explainer to mean you have to write long, analytical comments. Express yourself however you want! Authors love hearing about how much you liked their work no matter how you choose to tell them.
This is meant to be a guide for people who want to do this, but don't really know how, because they find it difficult to express themselves, or don't know what authors like to hear. If that's you, let's continue below the cut!
EDIT: Also, here's a post by bourbon-ontherocks about how to add a floating comment box to AO3, in case you want to grab quotations you like as you go! Thank you @flowersforthemachines for finding it <3
Why should you do this?
because it's fun!
because you really like someone's work!
because it's motivating for an author, and can sometimes inspire them to post more, or re-post old stuff!
because it's usually a great conversation starter! Some of my best fandom friends have been made in comment threads on AO3!
What do authors want to hear about in long comments?
Many things, but primarily:
What you liked about their work
Why you liked it
You can show them what you liked by quoting their work back at them (I find it useful to put quoted text in an indented section; the html code for this is <blockquote>text</blockquote>), talking generally about which events or characters you enjoyed most, or, if you have thoughts about it, what you liked about the structural parts of their work-- i.e. plot, pacing, sentence structure, etc.
As for telling them why you liked something, getting into emotional reactions is great for this-- you can tell them where you laughed, or cried, or where you felt moved by something they said. If you found the way they had the characters talk or think realistic or relatable, tell them that! If you want to, authors love hearing that people noticed them foreshadowing events, or planting clues within the text.
You can also tell them where you were when you read it, or your overall experience -- did you read at 3 am? Do you have school or work tomorrow, but you just had to finish reading? Did you read their fic in the club? Tell them! And if you're feeling especially brave or you want to share, feel free to tell authors how or why you related to the text. If it makes you uncomfortable, don't worry about it, but those are some of my favorite comments to reread.
How do I comment on specific parts/lines of a fic?
Okay, a demonstration! I am going to pretend to analyze my own work here, for convenience:
He dreams of it, ceaselessly. It seems to echo throughout his days; he cannot let his mind drift too far, lest he fall back into it. He has to cling to this world, like a drowning man to a raft, though it rarely feels worth the effort.
Okay, so let's pretend I read this section and I really liked it. If you want to talk about that, stop and consider what about it was special, for you. Maybe I thought that the third sentence really captured what it felt like to be depressed, or the second reminded me of something that happened in my own life. I might say:
"I really loved this part! I found the way you wrote about [the character's] feelings in this paragraph so relatable; I think you captured exactly what it feels like to feel consistently hopeless."
You can also take a look at the way the author has written the sentences-- try reading them out loud. If I think the last line has a nice sound, I might say something like:
"I liked how you chose to structure the last sentence! The grammar you used gave it a really nice combination of phrases and stops. I loved how it sounded, and the rhythm the punctuation created."
Another thing you could talk about was how the sentence or paragraph made you feel about the character. If that section made me feel particularly sorry for them, or if this was the point in the story where I connected strongly with them, I might say:
I think this part worked really well to demonstrate how [character] felt! You communicated the pain they were in really effectively, and I was very connected to them, during this part.
You can also just express your excitement! I love great writing, and sometimes I just keysmash, or put a million exclamation points, or say AHHHHHHH!!!
Okay, that's all the advice I have for writing long comments, for now! If you have any questions, or would like to talk about similar things with me, please feel free to ask or DM me at any time! Thanks for reading, and happy commenting!