First, # 3 is "Say it ain't so." It is closer to "I am pretty certain that this *is* true, but it makes me unhappy/sad/disappointed. I hope that you can tell me that I'm wrong and it isn't true. But I know that you're going to say that it is true." Second, all of those are fixed expressions and they are fixed informal expressions. I would almost never say "ain't" outside of that type of fixed expression (there are a few others not on that list). I rarely use one of those expressions, but when I do, it calls attention to itself and the expression. I would only tend to use it in informal contexts and only do so very deliberately. I think it would be good for you to understand those phrases, but I don't think you should try using them until you're very comfortable with English.

Not every native speaker uses "ain't". It's street language and so it depends on your upbringing, habits, and the image you want to project. Non-native speakers, by definition, won't say "ain't" as a result of their upbringing and probably nor their habits. I was brought up in East London where "ain't" was rife, but my Scottish parents beat it out of me, so to speak. As for image, in my opinion, it always sounds bad when a non-native speaker uses street language badly or even if they do so well but then make a mistake which a native speaker would never make.

I wouldn't use "ain't" as a learner of the English language. You have some excellent suggestions though. Normally, "ain't" is usually used as a replacement for "have not" - I ain't gonna do that (I am not going to do that, or He ain't come today. (He has not come today) This list is as 'socially acceptable' as you can get. You could give me this list, dare me to say them in a party full of fancy educated people, and I could slip them into conversation. It would be gimmicky, but they wouldn't ask why I suddenly misspoke and used the wrong grammar. Nobody would correct me. But it's gimmicky. The first one is a great song "You Aint Seen Nothin Yet" by Bachman Turner Overdrive "Say it aint so" is in the song "All the Small Things" by Blink 182 Here's the correct versions of you seven sentences 1. You haven't seen anything yet. / You haven't seen nothing yet. (A bit more emphatic, informal, emphasizing the unexpected.) 2. You haven't heard nothing yet. (Same as 1. The point being is that what you are about to hear is completely different and transformational to your past experiences. 3. Say it isn't so. ("Please tell me you are lying." I also like your translation. It's also used as a "Of course. You're speaking things we already know." Like, in a thunderstorm, "It's raining outside." "Say it ain't so..." It's a polite way to say they are speaking the obvious truth, that their sentence was a waste of time. 4. Isn't that the truth. (Meaning: I completely agree with your observation.) 5. That just isn't so. (Meaning: That just isn't true. / You're wrong. Your statement is mistaken.) 6. Two out of three isn't bad. 7. If it isn't broke, don't fix it. I think you can use these forms of "ain't" if you are being conversational, using these phrases exactly. You are effectively putting quotes around the entire phrase, not using "ain't" in daily sentence construction. Now I am going to listen to that Bachman Turner Overdrive song, I love their stutter on ba-ba-ba-baby.

I agree with Michael. Just avoid it unless you are writing song lyrics or just want to sound goofy.

'Ain't' is very commonly used by native speakers. You are correct it is informal, but there is nothing wrong with using it in an informal conversation. I ain't got a problem with using ain't, and I don't see a problem with using ain't. But I would advise against using it in formal situations. However it is commonly used in Hiberno English in many situations.