1.All the students must(должны) pay for their education, examinations, books, the use of libraries.All the students had to pay for their education, examinations, books, the use of libraries.
2. Not many children from the working families are able to(могут) get higher education.
Not many children from the working families were able to get higher education.
3. She cannot (не может)do this work in time.She could not (couldn't) do this work in time