"Is the milk sour?"
Translation:O leite é azedo?
Is the milk sour? = O leite está azedo?
Is the milk sour? = O leite é azedo? (when it's a permanent characteristic/feature of a certain milk, like if you're asking about an exotic mammal's milk)
Is it the sour milk? = É o leite azedo? (like if you know there's a bottle of sour milk in the fridge and now someone is offering you a suspicious glass of milk)
É o leite azedo = it is the milk sour? ...sounds weird. O leite é azedo = the MILK is sour? ....the subject changed. HOWEVER, the use of é signifies permanence. Example "he is(é) tan" (because he is latino) VERSUS está signifying non-permanence. Example "he is(está) tan" (because he is white and got a suntan). Milk however is usually not permanently sour. Which is why é sounds weird. This is my understanding and i'm not 100% confident. But it is a good place to start.