it must be 接動詞ing表示的語法結構為情態動詞must 加進行時be doing ，中文意思是“肯定正在做某事”例如：It must be raining outside. I heard the ticking of the roof.
it must be 接動詞ing表示的語法結構為情態動詞must 加進行時be doing ，中文意思是“肯定正在做某事”例如：It must be raining outside. I heard the ticking of the roof.