I've understood about -ul for masculine and -ua or -a for feminine. But -le?
-le is used for feminine plural and represents the determinative article. Ex: the flowers/las flores = florile
Lapte isn't plural though. It is asking for "the milk" not "the milks." Why is the plural article used here?
Let me preface by noting I'm just a learner, but lapte isn't plural or even feminine, rather, as I understand it, neutral (substantiv neutru), which in Romanian is treated as masculine in the singular form and feminine in the plural. That being said, the definite article inflection of masculine nouns ending in -e is formed adding le, as lapte -> laptele; rege -> regele; frate -> fratele; iepure -> iepurele. Hope this helps.