What characteristic differentiates an imperfect authentic cadence (IAC) from a perfect authentic cadence (PAC) in tonal harmony?

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Imagine two kinds of cadences that both land on the tonic, but with different levels of finality. A perfect authentic cadence is a clean, strongest landing: the progression moves from V to I with both chords in root position, and the outer voices line up so the soprano ends on the tonic. That precise bass-to-tonic move in the bass and the stable voicing give it its decisive feel.

An imperfect authentic cadence also ends on I, but something isn’t as resolved as in the perfect version. The bass does not complete the V-to-I motion in the bass line, so the cadence sounds a bit softer or less final. This can happen if the final I isn’t in root position or if the soprano doesn’t end on the tonic, but the key differentiator is that the bass doesn’t move from dominant to tonic.

So, the distinguishing feature is the bass not moving from V to I in the bass, which marks an imperfect authentic cadence.