3 Forces it swell above its wonted mode,
swell > [to swell] wonted > customary mode > way of being; hence: level
4 And largely overflow the fruitful plain,
largely > copiously; freely
5 That all the country seems to be a main,
That > [So that] main > {Broad expanse of water}
6 And the rich furrows float, all quite fordone:
fordone > ruined
7 The woeful husbandman does loud complain, 8 To see his whole year's labour lost so soon, 9 For which to God he made so many an idle boon.
idle boon > useless prayer