There are several modes and artificial scales that only change one note in a major scale, and this is one of them. To make simple minor, “simply” lower 3. The most basic example is C simple minor, which simply flats E: C D Eb F G A B C. “Simple minor” is my name because it is descriptive, but you will see the same scale if you read about jazz minor or ascending melodic minor.
Everything below is extremely advanced and will only make sense to very advances musicians.
In traditional theory what I call “simple minor” is called “melodic minor”, but there is a nasty twist. Traditional thinking only drills this scale ascending, so when theorists talk about “descending melodic minor”, it is the same thing as natural minor, a completely different scale.
Melodic minor, as defined traditionally, is not one scale. It is two scales, simple going up and natural going down. You would think that simple minor is not used going down, but it is used going down all the time. To this moment I do not understand how this quirky theoretical idea started.
The result is an insanely confusing and useless combination of the two scales, together called “melodic” minor. This in no way corresponds to reality in music and causes students an endless amount of confusion.
To make things worse, jazz players use this same “simple minor” scale ascending and descending, as I teach it, but they call it “jazz melodic minor”. In fact, you will see on “jazz minor” most of the time. But then they use this scale over a dominant by picking a jazz minor scale that is 1/2 step higher than the root of the dominant.
Example, play Ab [melodic] jazz minor (simple minor) over the shell of a G7 chord, but omitting the 5th because it clashes with the scale. Often only GF is used, or only BF, then when this is notated the scale is written differently, using B instead of Cb. All the notes in the scale are notated so that the scale notes fit logically into the chord structure.