The bass line
Hear the lowest voice of chords and write a bass line in the bass clef.
The lowest voice
The bass is the lowest voice in music. It sets the foundation of a chord: most often the bass holds the chord's root. So once you hear the bass, you almost know which chord is sounding.
The ear is not used to picking out the lowest voice: attention drifts to the melody on top. Start by singing the bass of the chords in your head.
Chords I – IV – V – I in C major
The same progression, bass only: C – F – G – C
Practice: Where is the bass?
Practice: A stepwise bass
How the bass moves
The bass moves differently from a melody: it leaps a lot, by fourths and fifths, from tonic to dominant, from subdominant to tonic. A fifth down and a fourth up lead to the same note.
Write the bass in the bass clef: C4 sits on the first ledger line above it, C3 in the second space from the bottom.
If the bass leapt and you're not sure where, check the tonic, the dominant and the subdominant. In simple progressions the bass is almost always on one of them.