So, we should save this useful river. (Imparative)

The Padma is one of the mightiest rivers of Bangladesh. (Positive)

= Few/ Very few rivers of Bangladesh are as mighty as the Padma.

It is a very turbulent river. (Exclamatory)

= What a turbulent river it is!

When it is winter, the river remains calm and tranquil. (Simple)

= In winter, the river remains calm and tranquil.

But during the rainy season, the river assumes a terrible shape. (Compound)

= It is the rainy season and the river assumes a terrible shape.

Everybody knows this. (Interrogative)

= Who doesn't know this?

The river is used for different purposes. (Active)

= People use the river for different purposes.