1.We played the piano yesterday.
-) We didn't play the piano yesterday.
?) Did we play the piano yesterday ?
2. She bought a ball for her boy yesterday.
-) She didn't buy a ball for her boy yesterday
?) Did she buy a ball for her boy yesterday?
3. They studied hard last semeter.
-) They didn't study hard last semeter.
?) Did they study hard last semeter ?
(-) : câu phủ định , (?) : câu nghi vấn