The use of standard Japanese "が" and "は"

"が" and "は" are the two most commonly used auxiliary words in Japanese, and the difference between them is a difficult problem for Japanese beginners. I went through a number of Japanese textbooks and grammar books, which details the difference between the two, such as "が" should be how to use, "は" should be how to use, and so on, very specific. But I have such a feeling: these introductions, although very detailed, very specific, but also very correct, but it does not seem to grasp the difference between the "essence." Here I would like to use a very simple example to show you how to grasp the difference between the "essence."

Assuming that Zhang San and John Doe were walking in the woods, Zhang San suddenly saw a bird on the branch (note that at this time John Doe had not seen, this is the first key), so Zhang San with his finger to John Doe, and said:

あそこに Bird がいます. (There is a bird there)

Zhang San with a finger, so John Doe also see (this is the second key place), Zhang San's eyes are very good, can distinguish the species of this bird, he continued:

あの Bird は parrots parrots ですので. (The bird is a parrot)

This is the end of this example, and now I'm going to summarize what the "essence" is: if there's nothing in the conversation's mind (the first key), you need to use "が", and once you talk to the other person's mind that something has already appeared (the second key point), you need to use "は".

Finally, I think it is necessary to emphasize one point: the textbooks, grammar books mentioned above are correct, of course, you must not use the content of my post to deny those content. I'm just doing a little supplement.

The use of standard Japanese "が" and "は"