3. Their homework / finish/ her/ since Monday.
Their homework has been finished by her since Monday.
4. A new bridge / be / build / next year.

A new bridge will be built next year.

5. This letter / send / two days ago.

This letter was sent two days ago.

6. The car / buy / last year.
.. The car is bought last year.
7. This bicycle / use / for a long time.
This bicycle has been used for a long time.
8. Millions Christmas cards / send / every year

Millions Christmas cards are sent every year.

3. Their homework / finish/ her/ since Monday.
=> ……their homework has been finished her since Monday 
4. A new bridge / be / build / next year.

=> a new bridge will be built next year.

5. This letter / send / two days ago.

=> the letter was sent two dáys ago

6. The car / buy / last year.
=> the car was bought last year
7. This bicycle / use / for a long time.
...=> this bicycle has been used for a long time
8. Millions Christmas cards / send / every year

=> Millions Christmas cards are sent every year .