Change the structure as in the example. This is my chair.

За my, your, her, his, its, our, their всегда стоит существительное. 

 Mine, yours, hers, his, its, ours, theirs ставится, если существительное опущено. Формы надо уяснить.

1. This is his car. - This car is his.

2. This is their dog. - This dog is theirs.

3. This is her key. - This key is hers.

4. This is your sandwich.- This sandwich is yours.

5. This is my cup. - This cup is mine.