must must must+5(have7) 猜测90%肯定 注：肯定用must 疑问否定用can,can't can can can't+5(have7) 猜测90%不可能 may may may+5(have7) 猜测50%有可能 注：肯定用may,could 疑问否定用could,couldn't could could ...