Welcome to the forum, Flavius.

Hey all I am really sorry I have enjoyed many stories on the site but am ashamed to say have never reviewed one. This is because though having enjoyed the story I would not feel that I am qualified to pass judgement/comment or even be critical of any of the work which individuals strive to produce for the delight and entertainment of others.
You are quite right to be ashamed! Most writers are glad just to have a simple "Enjoyed it" review. Don't worry about making critical reviews, just comment on the ones you like.

Reading the other reviews you'll soon discover that making critical reviews is a vocation in itself. Sometimes these reviews are better than the stories.

If you get the inclination to write a paragraph-long review, you'll most likely make a writer's day!