He bought a violin a week ago. From his letter I knew that he had bought a violin a week before
　　
　　B before 可表示不确定的过去 , 而 ago 则不能 : Have you been there before?
　　
　　20.very, much, very much
　　
　　very 可修饰形容词及副词 , 不能直接修饰动词 , 修饰动词时 , 要用 much 或 very much;much 除了修饰动词外 , 还可修饰形容词或副词的比较级及过去分词 ;very much 也可修饰过去分词 :
　　
　　The novel is very good./ This novel is much better than that one. / Her Japanese is improved very much.
　　
　　21. 像 afraid, alive, awake, asleep, alone 以“ a ”开头的形容词 , 一般不能用 very 修饰 .
　　
　　22.deep 与 deeply 都可作副词 , 但意义不同 :deep 指的是深浅的“深” , 如 :dig deep.Deeply 的意思是“涂地、深刻地、深厚地” , 有引申含义 . 如 :We are deeply moved by his deeds.
　　
　　23.high 与 highly 都可作副词 , 但意义不同 .High 指的是高低的“高” , 如 :jump high;highly 的意思是“高度地 , 高贵地” , 有引申含义 . 如 :She is highly praised.
　　
　　24.not 是副词 , 不能直接修饰名词 ,no 是形容词 , 能位于名词前作定语 , 有时 no=not any. 如 : I have no brothers=I have no any brothers
　　
　　25. 某人或某物在与同类比较时 , 某人或某物应排除在被比者之外 :Shanghai is larger than any other city in China. Shanghai is larger than any of the other cities in China .
　　
　　26. 用最高级时 , 主语本身应包括在最高级所指的范围之中 .He is the oldest of all the classmates.