THAY ĐỔI- thêm vào will, have- ed, be-ing,

                          because scientists had  warned .

because scientists have  warned .

because scientists           warned .

because scientists were  warned .

because scientists   are   warned .

     because scientists  will  be  warned .

        because scientists will        warn .

          because scientists               warn .

        because scientists        are   warning .

                  because scientists        were  warning .

       because scientists  have    been  warning .

       because scientists  had      been   warning .