Những mà to be phải giữ nguyên là:
*will be/won't be
*can be/can't be
*may be/may not be
*would be/would not be
*might be/might not be
*could be/couldn't be
*must be/mustn't be
*should be/shouldn't be
*have to be/have not to be
*has to be/has not to be
*ought to be/ought not to be
*used to be
CHÚC BẠN HỌC TỐT:)))))))
HAVE A NICE DAY~