んです - んだ - Struggling with the practical use / difference with masu form

Question

Mina-san konnichiwa.

I am struggling with the ndesu - nda form. I understand that in most instances this is used to ask or give more context / an explanation. In practical use I don't understand the difference between questions with or without this form.

For example: nani wo shite imasu ka? What are you doing VERSUS nani wo shite iru ndesu ka? What are you doing / What is it that you are doing?

Isn't it obvious that in the first question I also ask an explanation / reason / context? What is the difference? I have the same question for the answer part.

Watashi wa terebi wo mitte imase - I am watching TV VERSUS watashi wa terebi wo miru ndesu. I am watching TV / It is that i'm watching TV.

I really don't see the difference in meaning; I am sorry for this. Hopefuly someone can help me.

どうも ありがとう ございます

Answer 1

I’ll leave bulk of explaining 何をしていますか vs 何をしているんですか for someone else.

Though I will point out that 何をしていますか is in no way asking for an explanation. All that’s being asked is “what are you doing?”

何をしているんですか on the other hand might be asked in a context where you were expected to be doing one thing but you appear to be (or actually are) doing something else. For example, you might be on the job and you’re supposed to be rebuilding the engine for a truck. Instead, the foreman finds you watch youtube videos on you iphone. In that case, your foreman might ask 何をしているんですか. Maybe you’re goofing off; maybe you’re researching videos that walk you through the assembly.

Consider another scenario: your mom walks into your room and out of pure curiosity asks 何をしているの. You could answer テレビを見てる.

In another scenario, maybe your little sister keeps bugging you to do something with her and you’re getting annoyed. In English you might say, “can’t you see I’m watching tv?”. In Japanese, you might say テレビを見ているんだ

Answer 2

In addition to the point about context/more explanation, there is another difference between んです and just the ます verb ending:

んです　is a little more casual than ます. If you use only ます all the time, you sound stiff and formal. This is perfectly appropriate for situations like work or talking to a teacher. However, if you want to close the emotional distance between you and the person you're talking to, んです is a grammatical form that sounds a little relaxed and friendly while still being polite due to the です ending.

You could use んです with a person you've gotten to be friendly with, or would like to be friendly with. A coworker or student or friend at your level, for example.

It could be used with a person above you, like a teacher or manager ONLY IF they are relaxed and casual with you. It is always appropriate to keep using formal ます verb endings/sentence endings with your superiors.