"This is not an apple."
Translation:Ez nem egy alma.
That would mean "This is also not an apple." See a reference here: http://www.hungarianreference.com/Misc_Grammar/negatives-nem-ne-sem-se.aspx
Az has two functions, so I guess is slightly confusing. this = ez; that = az; this apple = ez az alma; these apples = ezek az almák; that apple = az az alma; that chair = az a szék; those chairs = azok a székek
(Note that the definite article az/a is needed in Hungarian but not in English)