Answer:
a. Frogs could be heard croaking as we got near the swamp. (no error)
b. When one has good health, one should feel fortunate.
c. He sat down at his desk and began to write.
d. Hold the rifle firmly against your shoulder, and then take careful aim.
e. First, the surface should be carefully cleaned; then the glue should be applied.
f. No matter what political party one belongs to, one should listen to all candidates.
g. To be a legislator, a person needs more than intelligence; they also have to be a student of human nature.
h. The manager decided to offer free samples, and the next day the store was packed with customers.