Is "singular noun + and + singular noun" singular or plural?

When I type "[singular noun] and [singular noun]" in Word 2010, I find some interesting things.

1. 
   

   Bread and butter is essential. (the auto-correction warns me of subject-verb disagreement.)

   
   
   


2. 
   

   Bread and butter are essential. (OK)

   
   


3. 
   

   A knife and fork is on the table. (OK)

   
   


4. 
   

   A knife and fork are on the table. (OK)

   
   



5. 
   

   The needle and thread is new. (OK)

   
   


6. 
   

   The needle and thread are new. (OK)

   
   


7. 
   

   Cost and revenue is directly linked. (OK)

   
   


8. 
   
   

   Cost and revenue are directly linked. (OK)

   
   

I wonder how a native speaker would consider this phenomenon.

Answer

I would say that the general guideline is, if you are mentioning two items that normally combine to form a new single item, then using "is" is preferred; otherwise, "are" is correct.

So when you say

Bread and butter is essential

Then I assume you are talking about bread that has butter on it. (For example, "Bread and butter is essential when eating a big bowl of spicy soup.")
If you say

Bread and butter are essential

Then i assume you are talking about bread by itself, plus butter by itself. (For example, "Bread and butter are essential for any kitchen.")

For your other examples, Word may say "is" is okay, but I disagree with it. ("Needle and thread" is a bit of a borderline case, since they do go together, but I would say that they don't really combine to form a new single item.) So sentences 3, 5, and 7 are wrong.

Since using "and" normally means that you are referring to multiple objects, if you're not certain about some particular pair, you're generally best off using "are".