I Knew You Were Trouble
by Taylor Swift
is most likely in the Key of F#/Gb Major
Notes in F#/Gb Major:
A#/Bb, B, C#/Db, D#/Eb, F, F#/Gb, G#/Ab
Chords in F#/Gb Major:
Gb, G#m, Bbm, B, Db, Ebm, Fdim
This key can also be known as D#/Eb minor.
When it's used as a minor key, the minor key note is stressed more than the major key note. The same notes and same chords still apply.