導語：英語四六級考試中，單獨考察語法的題型不多，但是語法知識點卻貫穿整張試卷，這是由本站小編為大家整理的“大學英語六級基礎語法知識”供備考使用。
情態+動詞進行/完成進行時
情態動詞也可以和動詞進行形式或完成進行形式構成謂語：表示"應當正在……"，"想必正在……"的`意思。
【例如】
why should we be sitting here doing nothing?
she might still be thinking about the question you raised.
he can’t be swimming in such weather.
at the moment she may (might) be playing with her schoolmates.
we needn’t be standing here in the rain. we might take shelter in the hut over there.
情態動詞間或也可以和一個動詞的完成進行形式構成謂語，表示"應當一直在……"，"想必一直在……"這類意思。
【例如】
they are sweating all over. they must have been working in the fields.
they may have been discussing the problem this morning.
she couldn’t have been swimming all day.
worth的用法
worth，worthy，worth-while都為adj. 意為"值得"。
1. worth： be worth + n. 當名詞為金錢時，表示"…… 值得……"
be worth doing sth. "……某事值得被做"
The question is not worth discussing again and again.
2. worthy：be worthy of +n. 當名詞為抽象名詞時表示"……值得……"
be worthy to be done "某事值得被做"
The question is not worthy to be discussed againand again.
3. worth-while： be worth-while to do sth "值得做某事"
worth while： It is worth while doing sth
It is worth while sb to do sth.
典型例題
It is not ____ to discuss the question again and again.
h hy h-while h while
答案C. 由worth的用法可知，此句只適合片語be worth-while to do sth.。因此選C。
