Rewrite the sentences as required.（按要求改写下列句子）
Linda bought a new camera last week. (改成否定句)
Linda _____________________ a new camera last week.
The foreign teacher has stayed in our school for five months. (对划线部分提问)
________________ has the foreign teacher stayed in our school?
David could hardly believe his own eyes. (改为反意疑问句)
David could hardly believe his own eyes, _____________________?
Suzhou River was very dirty in the past, but now the water is quite clean.
Suzhou River ___________________be very dirty, but now the water is quite clean.
letter, to, Tina, already, sister, has, written, a, her (连词成句)
_____________________________________________________.