Translation:The child is eating the lemon and you are drinking milk.
i understood/wrote "l'enfant mange les citrons et tu bois du lait" and it was regarded incorrect. in a listening comprehension i think it's really difficult sometimes to get the "correct" sentence, especially if the "le/les" is not pronounced clearly... and the endings, well, I (!!) can't tell any difference between "citron" and "citrons".
It is the le/la/l' that conveys singular. And the les conveys the plural. This means that it is really important to practice one's oral understanding and skill to distinguish between le and les. The plural s does not change the pronunciation of the word, at least not unless it is before a word beginning with a vowel sound.
You can enter le and les and listen to several native voices saying them at www.forvo.com in order to practice your listening skill. It will become easier with time and practice!
The thing is, when you speak you do contract syllables in most languages. But as beginners we find it frustratiing that the pronounciation of words differ when they are spoken alone from when they are spoken together with other words. An as beginners we do need to practice our understanding of separate words as well as whole phrases.