#1
How do you find the key of a song? I used to know but I forgot.
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#2
Basically it's the main note of the song and whether it's major or minor depends on what notes are played. So for example, "Killing In The Name" is in the key of D minor, because the main notes of the song are D, F, and G; I believe it's the F note that makes a song in the key of D minor.
#3
a good rule of thumb is what chord the song ends with (definitely not always the case), but basically, it's the chord/note that the tensions in the song resolve to, the note the others lead up to. like for example, sing "do re mi fa so la ti..." and you're anticipating the last "do", right? that's because do is the tonic note of that scale, whatever key it's in, and you get a sense that the musical phrase is "complete" when you hit that note. so just listen for that completeness in the song (often on the last chord) and that's the note of the key. if it's a major chord, youre in a major key, minor chord, minor key. not all songs are necessarily in any key or stay in the same key, but for lots of popular songs, this is how it works.