I have 2 comment,because i'm not good with language,words and I'm shy, and because the author deserves my feedback....

I comment whenever I am able to. Sometimes I'm too much in a hurry and I forget to comment later. I want comments on my stories, so why shouldn't I comment on other stories? As an author, it means a lot to get a comment, and even greater to get a positive comment. :)

I comment if something sticks out to me like the story made me laugh, cry, want to hit one of the characters, rant, or something like that. I prefer not to comment if I don't have sincere feedback about the story. If I just like the story, but it doesn't stand out at all, then I am less likely to comment. If I really love the story, I might say 'I love the story, mention why, and more' or I will sometimes use 'I really love this story and I hope you continue it' or variations of that, but the story has to really make me want more and need more or invoke some sort of reaction. A lot I read are just ok reads.

i do comment on every chapter. get ready for a spam comment from me!
even if its a new story and only a foreword post, i still would comment.
nothing would make the author more motivated than receive a comment :)

Just like with the person below has said, when I read a story, I try to comment as much as I can, whether it be every chapter, or here and there while also including my thoughts on previous chapters I didn't leave one on. Even like now, it has pretty much become second nature to comment something. I do it to express my opinions/thoughts (if I found something funny, epic, or say, didn't like something a character did), and also because it is just in general awesome and nice to comment. Leaving a comment feels good, and I myself know it feels even better for the writer, plus if they respond back, you can get a conversation going and that can be pretty great!

Yep I do, almost all the stories I am subscribed to. Because I have an opinion, the authors deserve it for their hardwork, and it make them happy. I know that nothing makes an author happier than a sincere comment.