词汇、语法的掌握是应对大学英语四六级考试的基础，英语常用到的词汇，为备战英语四六级做准备。
it be essential that 后面一定用虚拟语气，且句子谓语动词固定为(should) + 动词原型;
it was essential that the applications forms send back before the deadline.
53. we _d_ our breakfast when an old man came to the door.
a just have had b have just had c just had d had just had
when 还可以表示①刚..就… (有动作先后关系)、②恰在此时;
44. i had just started back for the house to change my clothes _d_ i heard voices.
a as b while c after d when
45. i felt somewhat disappointed and was about to leave, _c_ something occurred which attracted my attention.
a unless b until c when d while
64. it was essential that these application forms _d_ back as early as possible.
a must be sent b will be sent c are sent d be sent
非谓语动词与独立主格：短语，句子(或者句子，短语)
1.当短语部分有独立主语，并且该主语不同于句子主语，这时短语结构称为独立主格。(它最明显的特点是短语部分有独立的主语)。
独立主格的结构1：名词(代词)+ 分词 [现在分词、过去分词]
现在分词表示主动的，正在进行的行为;过去分词表示被动的，已经完成的行为。 作题时要通过判断动作与名词(代词)之间的关系来确定是使用现在分词还是过去分词。当动作由名词(代词)发出时使用现在分词，而名词(代词)是受动者时则用过去分词。
54. silver is the best conductor of electricity, copper _b_ it closely.
a followed b following c to follow d being followed
49. all things _a_, the planned trip will have to be called off.
a considered b be considered c considering d having considered
call off 取消; cancel vt. 取消; 四级中的考点就是现在分词和过去分词的区别。
独立主格的结构2：with + 名词(代词)+ 分词 [句子当中作状语]
36. after the arab states won independence, great emphasis was laid on expanding education, with girls as well as boys _c_ to go to school.
a to be encouraged b been encouraged c being encouraged d be encouraged
as well as 和(相当于and); be encouraged不会考。
动词不定式有预示将来行为的含义。to be encouraged 将要被鼓励;
been encouraged 已经被鼓励完了，与主句的谓语动词相对;being encouraged 正在被鼓励
2.短语部分没有独立主语，则短语结构称为非谓语动词。
非谓语动词的结构：分词(分词短语)
句子的主语决定非谓语动词中的分词是用主动还是被动形式。如果是句子的主语发出动作，则使用现在分词;如果动作不是由句子的主语发出，则使用过去分词。