At the opening of this madrigal, the first line of text is sung by one voice, and the second line is sung by the entire group of four voices. All of this music is repeated, and then the third line of text is sung. What happens during the third line of text and what does not happen?

What happens during the third line of text is that new lyrics and a new musical motif is introduced. This answers to the madrigal's structure: aaB.

Explanation:

The madrigal is a type of composition that peaked during the Renaissance in Europe. It had a particular structure: aaB

This means the first two parts have the same musical motif, but different lyrics (that is why the 'a' is repeated). It can be sung by a soloist or the choir, like on the example mentioned in this question (first goes the soloist, then the choir). But then comes the B part: this means a new musical motif is introduced and the text also varies. Since it's different from everything that came before, it's named by another letter on the structure of the composition.

The third line is probably sung by the solo voice, without the group.