“否定式 + 比较级”语法结构

 The change in the patient’s appearance couldn’t have been more obvious. (Passage A, Unit 8)

  上例中的couldn’t have been more obvious是 “否定式 + 比较级”的结构，表示 “最……”的意义，意思是 “再明显不过了”或 “最明显”。
英语中“最……”的意义除了用形容词或副词的最高级来表示外，还可用 “否定式 + 形容词或副词的比较级”的结构来表示。其中“否定式”可以是名词短语的否定式，也可以是动词短语的否定式。例如：
名词短语否定式+比较级：
  Nobody else in the class is taller than Steven. (= Steven is the tallest in the class.)
  Nobody works harder than Mark does. (= Mark works hardest.)
  No doctor in the hospital was more patient than Dr. Mellinkoff. (= Dr. Mellinkoff was the most patient doctor in the hospital.)
  Nothing is more dangerous to people’s health than passive smoking. (= Passive smoking is very dangerous to people’s health.)
  No one in our family has a better memory than Jane. (= Jane has the best memory in our family.)
  Nothing is harder to believe than his story. (= His story is the hardest to believe.)
  Nothing wastes more time than chatting with no purpose. (= Chatting with no purpose wastes the most time.)
  No garden in the town is more beautiful than his. (= His garden is the most beautiful in the town.)

  动词短语否定式+比较级：
  The house couldn’t be more comfortable to live in. (= The house is the most comfortable to live in.)
  The food in that restaurant couldn’t taste worse. (= The food in that restaurant tastes the worst.)
  The story I read last night couldn’t be more interesting. (= The story I read last night is very interesting.)
  Children couldn’t be happier when they are close to nature. (= Children are very happy when they are close to nature.)
  This is a question that couldn’t be more important. (= This is a very important question.)
  He couldn’t have worked harder when he was in the university. (= He worked very hard when he was in the university.)
  I couldn’t have spent more time writing this book. (= I spent a lot of time writing this book.)
  There couldn’t have been more people in the square that afternoon. (= There were a huge number of people in the square that afternoon.)
  注：动词短语的否定式中常包含情态动词can / could。用于这种情况的can / could主要表示可能性。表示现在时间通常用can + 动词原形；表示过去时间通常用could have + 过去分词。