Ка_Бэ все записи автора Не могли бы Вы объяснить употребление артикля?

1. The door is locked. Where is the key?
2. Is this a tie? – No, it is a tie, it is a shirt.
3. Is this a cat or dog? – This is a dog.
4. A newspaper is on the table.
5. Tom lives in a big house not far from the office.
6. Have you finished reading the book, which Make give you?

Заранее благодарю.