C
The subdominant note is the fourth scale degree of any key, for example in C major the subdominant note is F since the scale goes C D E F G A B C.
The subdominant is the 4th scale degree. In G major, that's C.
G
Yes. Actually it's spelled "subdominant" but that's just getting picky.
The subdominant note in a key is the 4th. In G minor, it's C.
C
The tonic is the 1st note in the scale. For example, in the G major scale, G is the tonic.
D major(D,F#,A,D) for sub domonant, and E major(E,G#,B,E) for dominant
In a major key, the dominant (watch your spelling) is the 5th, and the subdominant is the 4th scale degree. So, in the key of C, F is the subdominant and G is the dominant.
The tonic note of any diatonic scale is the first note (starting note) of the scale. In the case of G-flat major the tonic note is G-flat!
The fifth note of C major scale is G. The fifth note of any scale is always called the dominant.
The subdominant of D major is G major.