11) Many forests were cut down in the past, so many species of animals and birds were destroyed. If forests

Решение:

Для продолжения предложения используем грамматическую конструкцию условного предложения (Conditional Type 3), так как речь идет о нереальной ситуации в прошлом и ее результате.

1. If forests had not been cut down... — первая часть условного предложения в Past Perfect Passive (had not been cut down), так как действие не произошло в прошлом.
2. ...many species of animals and birds would not have been destroyed. — вторая часть предложения в условной форме Past Conditional Passive (would not have been destroyed), что соответствует нереальному результату в прошлом.

Ответ: Many forests were cut down in the past, so many species of animals and birds were destroyed. If forests had not been cut down, many species of animals and birds would not have been destroyed.