英文簡單句子文法一問

D. I can't say is a more appropriate answer.

"Whether it is true" is the indirect speech of the question "Is it true". Because it is an interrogative (疑問句) and answer A and C are affirmative statements (肯定語), answer A and C must be wrong.

I don't doubt implies that I am sure. Though grammatical, it is not logical to be followed by an interrogative. The better sentence is

I don't doubt that it is true.

I can't say implies that I am not sure.

I can't say whether it is true is grammatically correct and logical.


Some notes on the verb doubt.

Oxford Advanced Learners' Dictionary has this example:
I doubt whether the new one will be any better.
What this sentence means is that I do not believe that the new one will be better.

Come to think about it. Why do we need "whether/if" after doubt when "that" conveys the better meaning.

The meaning of "I doubt if she will come" is clear.
But what about "I don't doubt if she will come"? Does it mean I believe she will come or I believe she will not come?
If you write "I don't doubt that she will come" or "I don't doubt that she will not come", there is no ambiguity in meaning.

2012-01-18 07:21:55 補充：
I don't doubt = I believe

I don't doubt is affirmative, just as I am sure and I can say.
"I believe it is true or or" is illogical.

2012-01-18 07:22:19 補充：
"I believe it is true or not" is illogical.

2012-01-19 05:52:43 補充：
B is only correct in the following situation.

Whether it is true? I don't doubt (that it is true).

當然係B, 因為:

A. I am sure 我肯定
B. 我不會懷疑
C. I can say 我可以肯定
D. I can't say 我不可以肯定

所以

Whether it is true, I don't doubt
無論 (Whether) 是否真的, 我不會懷疑

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如果要用A, C & D 就要放前面:
A. I am sure it is true
C. I can say that it is true
D. I can't say that it is true

1它是否真的
a我肯定(意思前後矛盾,不是答案:只能說,我肯定它是真的/假的)I am sure is true/false
b我不懷疑(意思正確,無論真假,我都不懷疑)
c我說得出來(同a一樣,意思矛盾,不是答案,只能說:我知道它是真的/假的)I can say it is true/false
d我說不出來(意思重複了,1句不肯定,1句否,它是否真的,我說不出,只要前半句,意思完全一樣)