could in polite requests

[Mr. X]

Hello,

I know could is used in polite requests: Could you hand this to me?

But can it be used in statements as well? Can someone give an example so I can understand the context?

Mr. X

[bhaisahab]

Hello,

I know could is used in polite requests: Could you hand this to me?

But can it be used in statements as well? Can someone give an example so I can understand the context?

Mr. X

"I could have been a contender." "The film was quite good, but it could have been better." "Your son's exam results are not very good, he could do better."

[Mr. X]

"I could have been a contender." "The film was quite good, but it could have been better." "Your son's exam results are not very good, he could do better."

Thanks. I meant in present tense, can we always replace 'can' with 'could'? For instance: it is the best deal you can ever get. Can we simply replace can with could here? ... deal you could ever get. Can we do this for all present tense sentences?