Як утворюється паст симпл и паст континіес?

1)town

2)building

3)people

everyone agrees that unless tiger population is protected ,they will become extinct.if it were not the efforts made by international campaigns the extinction would already have become a fact.it is time to cooperate and help to animals otherwise the future will be sad.supposing there were no tigers left in the world: how would we all feel? !