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Consider these definitions for "could", from Could definition and meaning | Collins English Dictionary

Definition 14: You use could not or couldn't with comparatives to emphasize that someone or something has as much as is possible of a particular quality.

a) The rest of the players are a great bunch of lads and I couldn't be happier.

My question: What is the difference between sentence a) and sentence b)?

b) The rest of the players are a great bunch of lads and I couldn't have been happier.

My comment: To me, there is no difference. Because, to me, sentences a) and b) both read: "... and I couldn't be/have been happier (at any time)". Do you agree with me?

Definition 16: You say 'I couldn't' to refuse an offer of more food or drink.
[informal, spoken]

c) More cake?'—'Oh no, I couldn't.'

My question: What is the difference between sentence c) and sentence d) and which one is more formal?

d) More cake?'—'Oh no, I can't.'

My comment: To me, sentence c) is conditional: "(If you gave me one more piece of the cake), I couldn't finish it." Do you agree with me?
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Rizan Malik What is the difference between sentence a) and sentence b)?

a) The rest of the players are a great bunch of lads and I couldn't be happier.

The speaker is on a footfall team. They are practicing every day and competing once a week. The speaker is so delighted that he claims his team is perfect for him.

a) The rest of the players were a great bunch of lads and I couldn't have been happier.

There is most likely a misprint in that sentence that was not caught by the editor. It should be past tense.

a) The rest of the players are a great bunch of lads and I couldn't have been happier.
However, there is another reading that the speaker is talking about his recent past happiness when he was playing with this team. He is no longer playing with them. Maybe he moved away to go to university, but still keeps in touch with his old teammates and feels emotionally still part of the team. He is recalling a past emotional state.

Rizan Malikc) More cake?'—'Oh no, I couldn't.'My question: What is the difference between sentence c) and sentence d) and which one is more formal? d) More cake?'—'Oh no, I can't.'

Both are used.

c) More cake?'—'Oh no, I couldn't.'

The speaker is embarrassed that they would appear to be a pig. Or maybe they would be taking the last piece. It is a position of a social faux pas.


d) More cake?'—'Oh no, I can't.'
The speaker is on a diet and another piece would seriously violate her doctor's orders.

Rizan MalikTo me, sentence c) is conditional: "(If you gave me one more piece of the cake), I couldn't finish it." Do you agree with me?

No.

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Definition 14: You use could not or couldn't with comparatives to emphasize that someone or something has as much as is possible of a particular quality.

a) The rest of the players are a great bunch of lads and I couldn't be happier. The focus here is that I couldn't be happier right now, ie at present.

My question: What is the difference between sentence a) and sentence b)?

b) The rest of the players are a great bunch of lads and I couldn't have been happier.The focus here is that I couldn't have been happier at some point in the past..

My comment: To me, there is no difference. Because, to me, sentences a) and b) both read: "... and I couldn't be/have been happier (at any time)". Do you agree with me?

Definition 16: You say 'I couldn't' to refuse an offer of more food or drink.
[informal, spoken]

c) More cake?'—'Oh no, I couldn't.'

My question: What is the difference between sentence c) and sentence d) and which one is more formal?C seems more hypothetical than D.Neither seems formal. Formal is eg No, thank you very much.

d) More cake?'—'Oh no, I can't.'

My comment: To me, sentence c) is conditional: "(If you gave me one more piece of the cake), I couldn't finish it." Do you agree with me? Both seem conditional to me. Clive