改写句子。根据括号内的要求改写句子，注意标点符号。
1. I can’t sing this song now. (用yesterday改写句子)
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2. He spent about twenty-one hours in space. (就画线部分提问)
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3. He could run very fast then. (改为否定句)
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4. Could you swim then? (做否定回答)
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5. difficult, was, but, it, clever, Helen, was, (.) (连词成句)
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