Tag questions.

Hi, LCChang

1. We know that it is very important, B) don't we.
When we form tag questions, it’s important to recognize the subject noun, modal words (if any used) and the verb. If the first part of the tag question is positive, the tag to follow must be negative. i.e.  “you do understand that, don’t you?”.  The tone of the positive tag was formed on my part to assume that you understand.

2. I believe that he was my best friend, wasn't he?. # 2 sounds a little odd.  The problem seems to be related to the verb” believe” and “was”.  If the sentence were “I think you are my best friend, aren’t you?”, then it would make more sense.  With this sentence, I assume you are my best friend, but something shakes what I assumed all along, so I use the negative tag to reaffirm.  Hope my explanation helps.
 

#2 is just a very odd question.

I suppose if you have amnesia, and someone refers to your childhood friend and says "You know who John is, don't you?" you could say "I believe he was my best friend, wasn't he?"... but I'm having a hard time trying to come up with other examples of when you could use that.

With #1, "isn't it" sounds more natural. If you use the "don't we," it sounds patronizing, as if you were speaking to children. With the "isn't it" you could be looking for the other person to confirm what you think to be true.

Both of these are very odd, and not very natural, choices to ask about tag questions... aren't they?

Yes, it's that "we" that sounds patronizing. We know we're not supposed to color on the walls, don't we? We know we're not supposed to hit our sister, don't we?

In contrast to:

We're all in agreement that this will be our first priority, aren't we?