Translation:He changes girlfriends every year.
Don't know if this is a pun on the word 都 with though, or you actually want to know why 都 is used in this sentence. xD
In a previous grammar section it is explained that 每 。。。 都 translates to "every single one". But this is usually not accepted as a correct answer. Very inconsistent.
Which one do you prefer? Be a single person or change girlfriends every year?
"He changes girlfriend every year" 'non-plural' should also be accepted (by Duo, not by his girlfriends)
It should be either his girlfriend or girlfriends. Your non-plural sounds wrong to me, but I'm not exactly the best english speaker.
sorry, I'm a native English speaker, and it is not correct to say "he changes girlfriend" (non-plural). It must be plural. AndrewKang4 is correct