How to translate "Est-ce que tu peux me dire combien il reste de lait ?" ? My trial: Could you tell me how much milk is left? Actually, I don't understand what "il" mean in the sentence, does it refer to somebody? Or, does it refer to the milk?
Yes, 'il' in these impersonal expressions doesn't mean 'he'. It really depends on the expression itself, but here are a few examples to make it clearer for you:
Il y a = there is
Il faut = one should, one must
Il pleut/neige/fait du brouillard = it's raining/snowing/fogging
There are plenty more others, but in your case 'il reste' simply means 'there is' or 'there remains'.
As far as I recall we don't conjugate the partative article after a negative (pas de lait), an expression of quantity (un litre de lait) and when there is a plural adjective placed before the noun des amis formidables but de bons amis. Good job it's not a complicated language!
I don't remember the rule extending to interrogatives and certainly my dear mother-in-law is always asking "Qui veut de la tarte?" Me! Me!