21. The giant alumroot was satisfied with its view of the river and the high drama of the garden but could not get used to its feeling of vulnerability and nakedness from the gaze of the vineyard.

Ans: 21. The giant alumroot was satisfied with its view of the river and the high drama of the garden, but it could not get used to its feeling of vulnerability and nakedness from the gaze of the vineyard.

The problem here is that with the compound verb "was satisfied . . . could not get used" uses "could" as a modal in the second half, but not the first half. This is not allowed.

One may change this into a compound sentence as above or

alternately,

21. The giant alumroot was satisfied with its view of the river and the high drama of the garden but was not used to its feeling of vulnerability and nakedness from the gaze of the vineyard.

change the second verb so that it corresponds with the "to be" in the first half.