Depends on my mood. But I feel like at least leaving a comment is better than nothing.

It's fanfiction it's supposed to be fun but if they want a critique or an actual review to help them improve then I might do one.

I don't usually comment on the fics that I read, but whenever I do, my main aim would be to leave something useful for the author, to cheer them up, give encouragements, etc.

I'd struggle to think of enough to say to make it a big review.

^ It doesn’t have to be big, it can be something like “I love this fanfic it is actually the best I don’t know any better fanfic. 10/10”

^Then what differentiates between it and a comment? Just giving the score?

^The difference is that it's directed at potential readers, not at the author.

^I don't think the majority of readers look at the reviews until they've already read the story and possibly reviewed themselves.