A could表示能力

1 could表示目前的能力时在间接引语中保持不变：

‘I can′t／couldn′t stand on my head，’he said．

他说：“我不能做三点倒立。”相当于：

He said he couldn′t stand on his head．

他说他不能做三点倒立。

2 could表示将来的能力时，在间接引语中可以保持不变，或由would be able结构来转述：

He said，‘I could do it tomorrow．’

他说：“我明天能做这事。”相当于：

He said he could do it／would be able to do it the next day．

他说他第二天能做这事。

3 条件句类型2中，主句中的could与A2中的变化相同：

‘If I had the tools I could mend it，’he said．

他说：“如果我有工具的话，我就能修补它。”相当于：

He said that if he had the tools he could／would be able to mend it．

他说如果他有工具的话，他就能修补它。

would be able在这里暗指所假设的情况有可能实现（可能他可以借到工具。）

4 条件句类型3中，could被转述时保持不变。

5 could表示过去的能力时，在间接引语里保持不变，或用had been able结构转述：

‘I could read when I was three！’she boasted¹．

她吹嘘说：“我三岁时就能看书了！”相当于：

She boasted that she could／had been able to read when she was three．

她吹嘘说她三岁时就能看书了。

B could表示允许

1 条件句类型2中的could在间接引语中可保持不变，或由would be allowed to转述：

‘If I paid my fine I could walk out of prison today，’he said．

他说：“如果我交了罚款，我今天就能走出牢门了。”相当于：

He said that if he paid his fine he could／would be allowed to walk…

他说如果他交了罚款他会被允许走……

2 表示过去时态的could在间接引语中可保持不变，或用 was／were allowed to或had been allowed to结构来转述：

He said，‘When I was a boy I could stay up as long as I liked．’

他说：“我小时候，我可以想呆到多晚睡觉就呆到多晚。”相当于：

He said that when he was a boy he could／was allowed to stay up．／He said that as a boy he was／had been allowed etc．

他说他小时候，可以／被允许爱呆到多晚睡觉就呆到多晚。