Harmonic, Melodic Minors Explained

There are three types of Minor Scales, Natural, Harmonic and Melodic.

When you’re playing a song in a minor key, if you want to play the associated chords (aka Harmony), you play the keys in the Harmonic Minor Scale.

When you’re playing the melody or the tune you use the notes in the Melodic minor scale.
For example, if you play a song in Am.
It has the notes:
Natural Minor: A-B-C-D-E-F-G
Chords will be:
Am, Bdim, Cmaj, Dm, Em, Fmaj, Gmaj
(I – ii(o) – III – iv – v – VI – VII)
A typical progression may go like:
Am – F – C – Em
However, the person playing the chords should play the notes in the Harmonic Minor which is just a sharped 7th:

This basically makes the 5/v chord a V. So the chord progression will go like:
Am – F – C – E
Try it, replacing the 5th with a Major instead of a Minor works!
Another example is Bruno Mars’s Grenade song:
Goes sort of like:
Dm – Am – Dm – Am (play this if you’re picking)
But the chords can go:
Dm – A – Dm – A (play this if you’re strumming / playing chords)