"The cat drinks its milk."
Translation:De kat drinkt zijn melk.
Actually cats are sort of an exception most people insist on seeing all cats as female and all dogs as male.
Even my mother said her and she to my male cat, while she knew. It is something that is hard to get out of your system.
(And I just happened to see an english gameshow this week and someone made a joke about the exact same subject. That as a kid they used to think that cats were female dogs or something. So it might be a thing that isn't unfamiliar to the UK either. Otherwise it is a big coincidence)
So cats are often referred to as female as a standard and dogs as male.
Well, in this context you can't use ervan, it's not a possessive. Ervan means 'from it, of it' ( https://www.vandale.nl/gratis-woordenboek/nederlands-engels/vertaling/Ervan#.W4CljKgaU0M ).
The only way of using ervan would be if you had already mentioned the milk and so don't want to repeat yourself:
Er is daar een kom melk. De kat drinkt ervan.