No one knows why this happened.

• Passive form: It is not known why this happened.

Fire, however, wasn't the only cause of destruction.

• Using "in addition": In addition to fire, there were other causes of destruction.

The San Francisco Ferry Building is the place that commuters catch their ferries.

• Using "at": Commuters can catch their ferries at the San Francisco Ferry Building.

Tropical forests are known to be essential to the balance of nature.

• Using "awareness": We are aware that tropical forests are essential to the balance of nature.

I disagree that teachers should discipline students physically if they misbehave.

• Using "approval": I don't approve of teachers disciplining students physically if they misbehave.