1. This umbrella belongs to who? in theory this is possible or you can say, to whom does this umbrella belong? 2. Yes, he is being nice to you at the moment.

1. Grammatically, both should be "whom". But, when the preposition, "to", is so far from the object, people don't notice that they are using the wrong form. Maybe it would be better grammatically to say, "To whom does this umbrella belong?" But no one really talks like that. Also, in English, we usually start with the question word. So, "This umbrella belongs to whom?" will probably only be used to make sure you heard correctly, or to express surprise or disbelief. 2. It depends on the context. Maybe he's always nice, but is being especially nice right now. Maybe he is not usually nice, but is right now. Most likely, he is either being nicer than usual or nicer than expected.

I have the same question here, but If I refer to person first. Is it right to say "do you know who this umbrella belongs to?" ?

English Grammar in Use 4th ed. by Raymond Murphy is excellent. There is a PDF floating around the internet. Unit 49C page 98. Who does this umbrella belong to? Unit 4E page 8. He is being nice ...

Who does this umbrella belong to? (O) This umbrella belongs to who? (x) In a question you use your voice to emphasize the key words in 0, Who at the beginning is stressed in x, the sentence reads like a statement not a question because there is no does, and to stress the who at the end is unnatural, but it is possible Better, To whom (note the m) does this umbrella belong? 2. Tom is being very nice to me at the moment. Is this mean he "just" nice to me at that moment? No, very does not mean just. Very means extra in this case, just means so-so