I   I   I   I
IV  IV  I   I
V   V   I   I

I      I      I     ii/IV V/IV    <--- ii-V-I to approach the subdominant
IV     IV     I     I
ii     V      I     I       <--- here we already have V-I, just add the ii before the dominant

I      V      I     ii/IV V/IV    <--- simple I-V-I- progression
IV     IV     I     V/ii          <- secondary dominant to approach the ii
ii     V      I     ii V      <--- introduce "turnaround" with ii-V-I of the tonic proper