三、句子的单复数互换
句子的单复数转换的规则是：对应句中的名词、代词、动词等，能变则变，不能变的，要么保留，要么去掉。要做到熟练掌握并运用这一规则，必须掌握名词的单复数的变化规则，代词对应的人称和单复数，动词的第三人称单数的变化规则等。在解题过程中，还必须具体情况具体对待。
eg. (1) The girl is from England .
→ The girls are from England .
(2) He is a polite and helpful boy .
→ They are polite and helpful boys .
上述例句中，划线的为能变的，没划线的为该保留的，加 号的a(an)为该去掉的。反之，在由复数变为单数时，则要根据句意在名词之前补上一个a (an)。
eg. They are English girls .
→ She is an English .
但是对于"There be"句型，单复数转换时句中的 a / an不能去掉，而要根据句式将其与some或any 互换。
eg. (1) There is a boat on the lake .
→ There are some boats on the lake .
(2) There isn't a tree behind the house .
→ There aren't any trees behind the house .
(3) Is there a pet dog in your home ?
→ Are there any pet dogs in your home ?
(4) There aren't any pictures on the wall of our classroom .
→ There isn't a picture on the wall of our classroom .
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