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In my opinion, they both share the same meaning of "~, isn't it". However, it
seems that they cannot be used interchangeably. For example, in the following
sentence, according to my mock exam textbook, ではないだろうか is correct for ＿＿ but
not だろうか .

> ラブレターや海外からの絵葉書をメールで代用するのはおそらく無理 ＿＿。
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2016-05-17T13:26:21.520
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2016-05-17T16:19:58.680
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question
grammar word-choice
Difference between だろうか and ではないだろうか
In the descending order of certainty:

  * ～だろう = "I think (some clause)." The speaker thinks the clause is probably (or almost certainly) true.
  * **～ではないだろうか** = "I think (some clause), isn't it?" The speaker thinks the clause is perhaps true, but he's less certain as compared to the first sentence.
  * ～だろうか = "I wonder whether or not (some clause)." The speaker doesn't know if the clause is true or false. **You cannot add 恐らく to this sentence** because the speaker is totally unsure.
  * ～ではないだろう = "I don't think (some clause)." The speaker thinks the clause is probably _not_ true.

>   * 彼は（恐らく）大丈夫だろう。 He must be okay.
>   * 彼は（恐らく）大丈夫ではないだろうか。 I think he is okay.
>   * 彼は大丈夫だろうか。 I wonder if he is okay. / I'm worried if he's okay.
>   * 彼は（恐らく）大丈夫ではないだろう。 He must not be okay.
>

In the example sentence, the speaker is saying "It's _probably_ impossible
(無理) to use e-mails in place of ...", so 恐らく無理ではないだろうか is the correct phrase.
恐らく無理だろう is equally fine. The existence of the word 恐らく is what makes ～だろうか
wrong. You can't usually say "I wonder if he is _probably_ fine." in English,
either.
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2016-05-17T15:40:51.703
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2016-05-17T16:19:58.680
2016-05-17T16:19:58.680
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answer