Question

Insert the appropriate form of the infinitive.
1. They seemed (to quarrel): I could hear angry voices from behind the door.
2. They are supposed (to work) at the problem for the last two months.
3. The only sound (to hear) was the snoring of grandfather in the bedroom. 

Answer 1

1. They seemed (to be quarreling): I could hear angry voices from behind the door.
2. They are supposed (to have been working) at the problem for the last two months.
3. The only sound (to be heard) was the snoring of grandfather in the bedroom.