A city is ... than a town. (big)
bigger
This village is ... of all. (small)
the smallest
Your example: An ocean / a river
...
Your example: A lake / of all
...
A mountain is ... than a hill. (high)
higher
Mount Everest is ... on our planet. (high)
the highest
The Pacific Ocean is ... in the world. (deep)
the deepest
... waterfall on Earth is in Venesuella. (big)
The biggest
A park is ... than a forest. (small)
smaller
Rio de la Plata is ... river in the world. (wide)
the widest
This river is ... now than it was in the past. (dirty)
dirtier
Our city was ... one hundred years ago. (clean)
cleaner
