it is /this is 的分別??

The words this, that, these and those are demonstratives. In spoken and
written English, demonstratives are generally used to ‘point to’
something in a situation. The demonstrative pronouns this and these
usually refer to something which is near to the speaker, and that and
those refer to something further away, e.g.:

A: Do you
like this scarf? It matches my coat.

It is a pronoun and
refers to those not human in nature. It may be in a picture or your
thing. So, the thing may not near to you.

I meet a cat
when I come here. It is a black and white in color.

簡單答：

當場眼見到或指得到出來的用this，

不是說現在發生的東西，不能在當場見到或指出的便用it。

前提:
This is a pan ->This is a pen
以下用此作解釋

It is a pen.
it是指死物的代名詞,只要是死物,要標示它就可以用it

This is a pen.
this有指方向的用途,通常指較近的,伸手可及的東西,和that相反

兩者的分別,it是一般的陳述,但this有強調支筆比較近的意思

可能的情況是,有個謎語的謎底是一支筆,問你謎語的謎底是什麼,只能答 It is a pen. 答This is a pen.一定錯,因為沒有一支實際的筆出現了