A D-flat is also an E-sharp in music. Technically, if you flat an E-sharp, you have an E, not a really flat D-flat.
The tonic of the D flat scale is D flat.
B flat note, C note, D note, E flat note, F note, G note, A note, B flat note.
On the B-flat clarinet, a concert D is the lowest note.
The enharmonic of a note is another note that sounds the same, so the enharmonic of d flat would be c sharp.
D sharp and E flat
The mediant is the third note of a scale so the mediant of D flat is E
E flat (or D sharp)
D sharp
The leading note of Db major is C.
Lower case b is not a note in music or on the piano, it denotes a flat note. It is normally after another note e.g. Ab Db. On the piano you need to play the black note to the left of the written note. For example the black note to the left of D is Db (D Flat). The black note to the right is called a D# (D sharp).
E
D