13. No one stopped those people cutting down the trees. I was suprised
-> I was surprised that no one stopped those people cutting down the trees.
14. People are seriously polluting the environment ( passive)
-> The envirmonet are seriously polluting.
15. Tet is a festival. Tet occours in late January or early February. (which)
->Tet is a festival which occours in late January or early February.
Xin hay nhất ạ.