'declarative' questions and 'interrogative' questions (When to use which?)

Declarative questions:

1. 
   

   'Boss' and 'Manager' can be used interchangeably?

   
   


2. 
   

   She is going there tomorrow?

   
   

AND

Interrogative questions:

1. 
   

   Can 'Boss' and 'Manager' be used interchangeably?

   
   


2. 
   

   Is she going there tomorrow?

   
   
   

So my question is when to use the above type of questions (declarative) and when to use the latter ones (Interrogative)?

Although I used to use them both in any situation but some time ago a native speaker pointed out that you can't use both of them in any situation. But that person also said that he doesn't know the rules regarding this, and just 'know it'. So any ideas when to use which structure of question?

Answer

Ordinarily you use the 'declarative' only when the question is 'echoic' - that is, when you are asking for confirmation or clarification of what you have just heard, often explicitly echoing the language your interlocutor has employed. In most cases you will stress a specific term which you find incredible or did not hear clearly.

A: Janet's going to France next year!
B: She's going to France?

C: Janet's going to France? I thought it was Jessica.

And in these circumstances, where you are questioning what you have just heard, you will not use the ordinary interrogative form unless you stress such a term—otherwise your interlocutor is apt to respond “Weren’t you listening? Didn’t I just tell you that?”

A: Janet's going to France next year!
B: Is it France she's going to? not Spain?
C: Is it Janet who's going to France, or Jessica?

But if you are not responding to what has been said, if you're just asking for information, you will ordinarily use the interrogative form.