Can I能用 could 例如：Can I have a cake?

Can I能用 could 例如：Can I have a cake? Could you help me with my lessons?Yes I can.中为什么不能用could I'm so glad you could come.为什么用could而不能用can? Could you tell me how I can get to the theatre?中的can能不能用could代替 can i.yes,you could.这个对吗,我知道如果用could问,回答不能用could Could you help me with my lessons?Yes I can.中为什么不能用cond How long can I keep the library book?为什么不能用could啊? could 和can在语法填空中该填哪个?怎么填?but that is all I can afford.为什么不能用could.有什么区别 Could you tell me （） A、how I can improve my English A、how I could improve my English选哪个 如果选A 为什么不能用could I wish I could fly in the sky,could 和can,为什么不用can,could 是can的过去式吗? I really want to know what we can do to help them 这里为什么不能用what we could I wish I could/can?后面应该用could还是can? 谁知道can能不能用虚拟语气.比如说 i could …… ,it would…… .还是只能用 i were able to …… I would if I could ,but I can't it was so black that i could hardly see anything.这里为什么用was 和 could?能用is和can吗 我能用（借）你的书吗?用英语翻译的十种说法 例如：Can I use your book? could you 和 can you 的区别?例如,Could you do me a favor?Can you give me some food? If I can和If I could都可以吧?