We have seen many cases of tonic expansions based on T and D. Sometimes, composers use what we call plagal expansion. In this case, chord IV or its substitute chords are involved.
Ex. 3.2a - the bracketed I-IV-I is a good example of plagal expansion of the tonic subsumed within a larger harmonic phrase unit that begins with a D function.
Ex. 3.3 - the I-IV-I-IV-I is a plagal expansion of closing tonic; note that the structural cadence is the perfect cadence at bs. 39-40, the final IV-I is not a structural cadence but part of a plagal expansion.