1. The major mode is a mode composed of seven tones, in which the stable tones combine to form a major triad. The tonic of the major mode and the third note above it are major thirds (such as do to mi), because this interval best describes the color of the major mode.
2. The flute can play all the keys in music. In music, the major keys include C.D.A.E.B, F sharp, C.F. B flat, E flat, A flat, D flat, G flat, C flat. tune.
3. It’s very simple. You first look at the sharps and flats at the beginning of the song (for example, if there is a sharp, it is G major, sou is used as do), and then look at the ending note at the end of the song. If so. sou that's G major. If it’s other notes, it’s a minor (for example, there’s a sharp in the front of this piece but the ending falls on mi, which is e minor)
If you don’t understand, you can buy a copy of Li Chongguang’s Basics of music theory, this book contains some simple music theory knowledge. This book is very useful if you want to learn music theory. I hope my answer will be useful to you! If you still don’t understand anything, you can ask me again.