Как это по-русски? 

 1. Whose dictionary is this? - This is my dictionary.

 2. Whose pen is this? - This is his pen.

 3. Whose children are they? - These are her children.

 4. Whose dog is this? - This is our dog.

 5. Whose house is this? - This is their house. 

 6. Whose letter is this? - This is your letter. (informal)

 7. Is that your purse? (formal)  - Yes, this is my purse.