This is a door: when you are referring to something that is near. It’s a door: when you are talking about the door

Question: when is a door not a door ? Answer: when it's ajar. (A JAR ) silly english joke.

"This is a door", you are near the door. Ex: This is a door that will oppen soon. But, "It's a door", it's a general sentence, you can use it in different contexts. Ex: It looks like a mirror but it's a door.

No difference. Без разницы. It is just a short way to writing as in it is in speaking . But if it says the door it means to open a specific door.

I agree with Laura This is a door: tells the reader that you talking about a specific door. It's a door: is general and the meaning depends on the context of the sentence.