These are getting harder fast. In third species counterpoint, four quarters are set against each whole note. My big lesson from the exercise I've posted below: 'f's in the cantus firmus make it difficult to write a counterpoint below. That's because you've got two adjacent dissonances: c-f is a 4th, and b-f is an augmented 4th. In the sixth bar, here, the only way I could see to get out of that bind was the cambiata formula.
It takes forever to mark, then tinker. It's hard to believe things are going to get still more complicated. (On the other hand, it's also hard to believe how much improvement I can observe in working through an exercise set. These Dorian mode exercises I've been posting are always the first ones in a set. It usually takes me about 1/3 as much time to complete the Ionian mode-- the last one in every set-- as it did the Dorian.)