"We saw tall trees."
Translation:Ми бачили високі дерева.
Singular tree is дерево, so 'We saw a tall tree' would be 'Ми бачили високе дерево.' Please notice that both cases here, дерева and дерево, are in accusative, which here look exactly same as nominative. And here in дере́ва the second syllable is stressed in pl. nom. and acc., while де́рева (1st syllable stressed) is sng. gen.