Difference between もう いい です and もう けっこう です


I'm going back over my notes from Japanese For Busy People. I no longer have
the books but I wrote out many of the examples. What I have not always done is
write down my teacher's explanations. Hopefully I can get an answer here.

In these two examples we see けっこう and いい used. I remember using けっこう for
saying one does not want any more tea (いいえ、もう けっこう です) but I don't know if it
is the same usage here. More commonly I use もう いい です. So seeing けっこう here has
confused me.

Could somebody explain the difference between these two examples please?

> 今 ペンが ありません。 えんぴつで も いい です か。  
>  はい、えんぴつで も けっこう です。
>
> 今 現金が ありません。 カードで も いい です か。  
>  はい、 カードで も いい です。




We are talking about two different (though related) shades of meaning of
「けっこう」 here.

> "no longer in need of ~~"

「もう **けっこう** です。」 means "I want no more ~~."

「もう **いい** です。」 has at least two meanings. One is the same as 「もうけっこうです。」, but
it is less formal than 「もうけっこうです。」. The other meaning is "(Something) is ready
to (or 'to be') ~~."

> "sufficient", "satisfactory", etc.

「えんぴつでも **けっこう** です。」 = "A pencil will suffice (if you have no pen)."

「カードでも **いい** です。」 = "A credit card would be no problem (if you do not have
cash.)" A store clerk would generally speak more formally than that and would
say 「カードでも **けっこう** です。」




We are talking about two different (though related) shades of meaning of
「けっこう」 here.

> "no longer in need of ~~"

「もう **けっこう** です。」 means "I want no more ~~."

「もう **いい** です。」 has at least two meanings. One is the same as 「もうけっこうです。」, but
it is less formal than 「もうけっこうです。」. The other meaning is "(Something) is ready
to (or 'to be') ~~."

> "sufficient", "satisfactory", etc.

「えんぴつでも **けっこう** です。」 = "A pencil will suffice (if you have no pen)."

「カードでも **いい** です。」 = "A credit card would be no problem (if you do not have
cash.)" A store clerk would generally speak more formally than that and would
say 「カードでも **けっこう** です。」

