Major 7th chords are commonly used in place of plain major, minor and dominant chords. The formula for maj 7th is ROOT-3-5-7 and the formula for Minor 7th are ROOT b3-5-b7. In the key of F in the Major scale this would be F-A-C-E. In the Minor scale it's Bb-D-F-A. In the key of G this would be G-Bb-D-F. In the key of D it's D-F-A-C. Dominate 7th chords (7), sometimes referred to as "dominant," play a very important role in chord progressions, The formula for a 7 chord is Root-3-5-b7. It has a natural 3, like a major chord, and a b7 like a min7 chord. In the key of E it is spelled E-G#-B-D. In the key of A it's A-C#-E-G. Practice spelling Maj 7th, min 7th and Dominant 7th chords for the other ten Keys. The attached song has an example of Maj 7th, min 7th and dominant 7th chords, one with bass, one without and then the complete song.