英语中如何将肯定句变成否定句?肯定句变成疑问句?

肯定变否定,就是在系动词后面加not,或者谓语动词前面加上相对应时态的情态动词,加上not,并且谓语动词用原形.比如：I am a doctor.I am not a doctor.He likes to play football.He does not like to play football.
肯定变疑问,　就是把系动词提前,或者是在句首加上相对应时态的情态动词,并且谓语动词用原形.比如：I am a doctor.Are you a doctor?He likes to play football.Does he like to play football?