Pelin, very good! The only error is in the first sentence. You can say "this relationship won't work" but not "this relation won't work." "This relation" doesn't make sense. (Relation is almost always used to mean "in relation to [something else]". Means "compared with." You're doing great!!

They look very good for the most part, but there are slight differences between the sentences: This relation won't work. --> This is not specific to a relationship between people and sounds awkward if you are, in fact, talking about a relationship. This relationship won't work. --> so this sounds more natural. I don't intend to stay here. I have no intention of staying here. --> These sentences are completely interchangeable. It's not like anything I have ever seen before. --> You could also say "It's unlike anything I've ever seen before." It's like nothing I have ever seen before. --> But both of these sentences work correctly :) There is something I didn't mention to you. --> This sounds awkward in conversation. I would drop the "to you" because it is implied in conversation. There is something I didn't tell you about. --> This is perfectly correct - Bravo! Then I have learned something I will never forget. Then I have learned something I would never forget. --> the "would" in the sentence seems out of place to me. "Would" in this context does not adequately replace "will" like how it would if I were to say "I would/will never do that" or "I would/will never forget that." I would change this sentence to "Then I have learned something unforgettable" I hope this helps!