The American Ornithologists Union considers the Green-winged Teal and Eurasian (or Common) Teal to be subspecies of Anas crecca. Most other sources, however, consider the two to be separate species, Anas carolinensis and Anas crecca. I tend to agree with the latter set, primarily because it provides another species for my life list. Eurasian Teals range as far east as western Alaska, so a few end up in Oregon every winter.
Here we see a male Eurasian Teal on the left, and a male Green-winged Teal on the right. Eurasian Teals have a bold white line along their sides, while Green-wings have a vertical white line at the side of the breast. Hybrids between these two species show both the horizontal and vertical lines. I personally don’t know how to distinguish female Green-winged Teals from Eurasians.
Here are the same two birds. While it is not obvious in these very distant photos, at close range we would notice that the pale lines surrounding the green eye patch are more pronounced on Eurasian Teal than on Green-winged.