Question

represent f1 and f2 generation by crossing a pure tall plant with a hybrid tall plant

Answer 1

Mendel started out with plants that "bred true". That is, when tall plants were self-pollinated (or cross-pollinated with others like them), plants in following generations were all tall; when the short plants were self-pollinated (or cross- pollinated with others like them) the plants in following generations were all short.
Hybrid, offspring of parents that differ in genetically determined traits. The parents may be of different species, genera, or (rarely) families.