So, we should save the useful forest. (Imperative)

Sunderban is one of the largest Mangrove forests in the world. (Positive) = Very few Mangrove forests in the world are as large as Sunderban. 

How nice the forest is! (Assertive) = The forest is very nice. 

As soon as I saw the tiger, I climbed upon the tree. (Negative) = No sooner had I seen the tiger than I climbed upon the tree. 

Tell him to tell a tale. (Passive) = Let him be told to tell a tale. 

Everybody knows this. (Interrogative) = Who does not know this? 

If we destroy mangrove forest, we will lose our ecological balance. (Simple) = By destroying mangrove forest, we will lose our ecological balance.