Chords I, IV, V and VI
Chords I, IV, V and VI are important as, between them, we can form any of the four cadences.
In a major key, it is normal for chords I, IV and V to be major chords. Chord VI is normally a minor chord. In a minor key, it is normal for chords I and IV to be minor chords. Chords V and VI are normally major chords.
The easiest way to remember this is to think of the table below:
|I||defined by key|
|IV||same as chord I|
|VI||opposite of chord I|
So, if the key were F major, then chord I would be F major, chord IV would be Bb major (same), chord V would be C major (always major), chord VI would be D minor (opposite).
And if the key were D minor, then chord I would be D minor, chord IV would be G minor (same), chord V would be A major (always major), chord VI would be Bb major (opposite).
Explanation of chords and how they are formed.