1 Its a long road. It is a long road. It is the longest road.
2 It is a good student. He is the best student. Hes the best student.
3 Students completed a difficult exercise. Students completed a difficult exercise. Students completed the most difficult exercise.
4 I bought a beautiful painting. I bought a beautiful painting. I bought a beautiful painting.
5 My room is very convenient. His room is more convenient. Her room is the most convenient.
6 Nick read an interesting book. Mike read a more interesting book than Nick. Tom read a very interesting book.
Удачи!