21.We have invited all the students in the school.
=>  all the students in the school. have been invited
22.They posted all the letters yesterday.
=> all the letters were posted yesterday
23.The police found two children in the forest. qkd
=> two children were found  in the forest by the police
24.They will hold a meeting before May Day. tld
=> a meeting will be hold before May Day
25.Mary is cutting the cake with a sharp knife. httd
=> the cake is being cut with a sharp knife by Mary
26.The teacher is going to tell a story.
=> a story is going to be told by the teacher