Final particles の / んだ


I just watched a video on this topic. I've been learning Japanese for roughly
2 years now and it still confuses me. I need some clarifications. What I've
been able to establish is:

  1. don't use の in genuine questions because it comes off... weird? It shows a surprise. You'd rather say なにしてる？ rather than なにしてるの？ and that applies basically to any question. 
  2. don't use んだ？in questions if you're a woman. Sounds rough. Use の or んですか
  3. use の and んだ/です to express "because..." without actually saying the actual word から. Usually in answer to なんで/どうした  
3.5 When you ask questions with なんで/どうした add の/んですか？to sound more natural.
Don't use んだ？if you're a female.

  4. んだ + けど (ですが） sets up the context for what we really want our speaker to pay attention to. The second sentences is more important. Can be translated as "and." You can ask questions with it too. 
  5. んだ/です can work as a new topic starter. "So...I did this or that" you want to be questioned about it/talk about it/brag about it even? 
  6. Don't state facts with んだ/です "I bought a new car." Simple fact. Nothing interesting to it. 
  7. logical assumptions. "oh, I see, so.. you're an anime fan" (The person keeps talking about it hence my assumption) 

Am I correct? Is there anything else to it? I'm pretty sure it all hinges on
context (like pretty much everything). Also, is the tone of my voice also
important here?

One more question. I've encountered ゲームを買うんじゃなかった translated as "I shouldn't
have bought the game" Is んじゃなかった a negation of んです？it really confuses me.  
Wouldn't it mean "It was because I didn't buy the game"




Explanatory-の is a very big topic and it's impossible to cover everything in a
single question, but your explanation seems OK to me for the most part. But
let me comment on some of your statements.

> You'd rather say なにしてる？ rather than なにしてるの？ and that applies basically to
> any question.

I'm not sure what you mean by "genuine", but "なにしてる?" sounds fairly blunt and
oppressing to me. It's almost like "What are you doing!?" yelled at a thief
you just witnessed at your home. "なにしてるの?" is much more common. This is the
most relevant question I could find: [The real meaning of の when asking a
question](https://japanese.stackexchange.com/q/70249/5010)

> don't use んだ? in questions if you're a woman. Sounds rough.

This may be an over-simplification. んだ in a "wh"-question is indeed fairly
rough, and even male speakers do not usually use it in reality (see above).
Perhaps んだよ is more acceptable, but it's still rough and uncommon. On the
other hand, strong or military female speakers sometimes use んだ in fiction.
And んだ in a yes-no type question is like friendly "huh?", and it can be safely
used regardless of sex (e.g. 知ってたんだ? = "(Oh,) You knew it?").

