—Could you swim at the age of 5 ? ——No,I____

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—Could you swim at the age of 5 ? ——No,I____
（选项略）
这是英语周报上的一道题,它的答案是 couldn’t .
但我曾经在另一本教辅上看过这样一个语法点：在问句中,could比can语气更委婉,但无论在肯还是否定句中,其答句都不能用could,而要用can.
请问如何理解?哪个说法是正确的?

其实,你所说的“委婉”用法,是could 用于一般现在时,表示请求或征询意见.
而can 还可表示能力,could 表示过去的能力.此句的意思是“你五岁的时候会游泳吗?”,表示能力,不表示委婉.
could的用法：
1.“能；会”,表示过去的能力.如 He could write when he was five years old.
2.“可以”,表示过去的许可.如 She said we couldn’t get into the room.
3.“能”,表示请求或征询意见,比can更委婉.如 Could you help me with English every day?答语常不用could,而要说成：Yes,I can./ Sure./ Certainly./ Of course./ All right./ OK./ I’m sorry…等.
4.“可能”,表示推测,用于肯定、疑问或否定句中均可.
○1表示对过去的推测.如 He said he couldn’t help her again.
○2表示对现在、将来的推测,但把握性比can小.如 – Where’s Bob?– I’m not sure.He could fight with others again.

could you swim at the age of six?i said to Jim,改为宾语从句 I could swim at the age of seven同义句 At the age of 10,I could swim同义句 i could swim at the age of ten为什么用at at the age of eight,Jim could swim改为同义句 I could swim at my age of five .同义句 Could your brother run at the age of two?—Yes,he____(can / c I could swim at the age of five.同义句转换 I （）（）to swim when I w He could not swim at the age of eight(改为同义句） He couli not __ Jim couldn’t swim at the age of five-------- ---------/ 动词应用The boy start learning.(swim) at the age of 5 i could swim _______the age of ten.