I found this question in my third-grade musicianship books and don't know the answer. Notice that the dominant triad contains the leading note and therefore needs special care in minor keys. Why?
In minor keys the 7th note in the scale can be a b7. As in A minor, the note will be G. The dominant triad used in A minor, often to return to the root, will be EGB, an E minor chord. This doesn't sound so decisive, so the G is sharpened. So now the dominant chord is spelled EG#B. This puts the leading tone a semitone below the root, which is a note not found in the minor necessarily.
With the sharpened leading note (e.g. G# in A minor), another issue is the augmented fifth from the 3rd to the 7th degrees of the scale, and (in the harmonic minor) the augmented second from the 6th to the 7th degrees. Don't write "unsingable" parts that use those intervals. The inverses of those intervals (7th to 3rd, or 7th to 5th) are less problematic so long as the next note lies inside the diminished intervals, but the leading note wants to proceed to the tonic, which is outside the augmented intervals.
With the flattened leading note (e.g. G natural in A minor), you weaken the natural tendency of the leading note progressing to the tonic, and the end result can be a chord progression that doesn't have a clearly defined key, but could be in the minor, relative major, or even some modal tonality.
Of course this isn't necessarily "wrong," so long as you know you are doing it and it is intentional - but it may lose you marks at "third grade" level, where the purpose of the exercises you write is to demonstrate that you can follow "the rules", not that you can break them.