Hi Kachon, Those sentences you listed are tag questions. The verbs are used in pairs, one being positive and the other is negative. The first part is to declare something, the second part is to ask for a response. Example, You ARE not very good at this (Negative,suggesting you are not good), ARE (positive) you? (question whether you agree or not. OR You are very good at this, are you? (Positive), aren't (negative) you? If you write : You are not very good at this? Then the meaning is incomplete because it seems you are making a comment and a question at the same time.

Without the last two words of either sentence, they are just statements: "You're not very good at this." "We're lucky." The point of questions like these is to express the speaker's own answer to the question while asking it. In the first example (which would often be considered rude), the speaker already believes the person is not good at the task they are doing and wants to express it. In the second sentence, the speaker believes that the people are lucky and is just looking for confirmation from whoever is asked the question. These types of questions try to reduce a more complicated question like "Are you good at this?" to a simpler yes or no decision. You can still answer them with a qualification, such as: "I'm not very good but I'm getting better." but the question is usually just looking for a yes or a no.

Search for "tag questions". There are plenty of sites which explain how these extra parts work.