「こ, か」, 「だろう」: Key Differences

When reading Japanese, you may come across sentences ending in ことか (koto ka) or だろう (darō).

Consider the following examples.

「合格の知らせを聞いて、彼はどれほどうれしかったことか。」
How delighted he must have been when he heard that he had passed.

「試験が終わったので、彼も安心しただろう。」
The exam is over, so he has probably felt relieved too.

At first glance, both sentences seem to express the speaker's thoughts about someone else's feelings.

However, they do not communicate exactly the same thing.

ことか (koto ka) expresses a strong emotional reaction or sense of wonder.

だろう (darō), on the other hand, expresses conjecture: a judgment based on information, circumstances, or general knowledge.

Understanding this difference will help you not only with JLPT grammar questions but also with reading passages in which the writer's attitude is important.

The Main Difference: Emotion or Judgment?

The simplest way to remember the distinction is this:

• ことか (koto ka): emphasizes how strong a feeling, experience, or situation is
• だろう (darō): indicates that the speaker thinks something is probably true

With ことか, the focus is on intensity.

With だろう, the focus is on probability.

What Does ことか Mean?

ことか (koto ka) is used to express strong emotion, such as joy, surprise, sadness, relief, admiration, or hardship.

Although the sentence may look like a question, the speaker is not actually asking for an answer.

It is a rhetorical expression.

Common Patterns

• どれほど ＋ plain form ＋ ことか
• どんなに ＋ plain form ＋ ことか
• 何度 ＋ plain form ＋ ことか
• 何回 ＋ plain form ＋ ことか
• どれだけ ＋ plain form ＋ ことか

Example 1: Effort

「日本語で自然に話せるようになるまで、どれほど練習したことか。」
How much practice it took before I could speak Japanese naturally.

The speaker is not trying to measure the exact number of hours spent studying.

The sentence communicates a powerful sense of effort and experience.

Example 2: Longing

「遠く離れた家族に会える日を、どんなに待っていたことか。」
How eagerly I had waited for the day when I could see my family again.

The main point is the strength of the speaker's longing.

Example 3: Repetition and Hardship

「大切な書類をなくして、何度探したことか。」
How many times I searched for the important document after losing it.

The sentence conveys frustration and repeated effort.

What Does だろう Mean?

だろう (darō) expresses conjecture.

The speaker does not state something as an absolute fact. Instead, the speaker reaches a probable conclusion based on the available information.

Its polite equivalent is でしょう (deshō).

Common Patterns

• verb in plain form ＋ だろう
• い-adjective in plain form ＋ だろう
• な-adjective stem ＋ だろう
• noun ＋ だろう

Example 1: Prediction

「空が暗くなってきた。もうすぐ雨が降るだろう。」
The sky is getting dark. It will probably rain soon.

The speaker predicts rain based on the appearance of the sky.

Example 2: Inferring Someone's Feelings

「彼は毎日遅くまで練習していたから、きっと試合に勝ちたいのだろう。」
He practiced until late every day, so he probably really wants to win the match.

The speaker infers the person's feelings from his behavior.

Example 3: Estimating a Situation

「この時間なら、駅の周りは混んでいるだろう。」
At this time of day, the area around the station is probably crowded.

The speaker makes an estimate based on the time.

The Difficult Point: だろう Can Also Sound Emotional

This is where many learners hesitate.

だろう expresses conjecture, but some sentences using だろう can also contain sympathy, admiration, or emotional involvement.

Consider this sentence.

「長い間努力して、ついに夢をかなえたのだから、どれほどうれしかっただろう。」
After working so hard for such a long time and finally achieving the dream, how happy that person must have been.

The sentence has an emotional tone.

However, its grammatical core is still conjecture.

The speaker did not directly experience the other person's feelings. The speaker imagines them from the circumstances.

Now compare it with the following sentence.

「長い間努力して、ついに夢をかなえたとき、どれほどうれしかったことか。」
How happy I was when I finally achieved my dream after working so hard for such a long time.

Here, the focus is not on probability.

The speaker highlights the intensity of the emotion itself.

Compare ことか and だろう Through Examples

Example 1: Passing an Exam

「合格通知を見たとき、どんなにうれしかったことか。」
How happy I was when I saw the acceptance notice.

The sentence emphasizes the intensity of joy.

「長い間勉強していたから、合格通知を見てうれしかっただろう。」
After studying for such a long time, the person was probably happy to see the acceptance notice.

The sentence infers a feeling from the situation.

Example 2: Reuniting With Family

「十年ぶりに家族と再会できたとき、どれほど安心したことか。」
How relieved I was when I reunited with my family for the first time in ten years.

The sentence emphasizes the depth of relief.

「十年ぶりに家族と再会できたのだから、きっと安心しただろう。」
After reuniting with family for the first time in ten years, the person must have felt relieved.

The speaker makes a reasonable assumption.

Example 3: Living in an Unfamiliar Environment

「慣れない環境で生活するために、どれほど苦労したことか。」
How much hardship I experienced while living in an unfamiliar environment.

The sentence emphasizes the difficulty of the experience.

「慣れない環境での生活だから、苦労することも多いだろう。」
Because it is life in an unfamiliar environment, there will probably be many difficulties.

The sentence expresses a prediction or general assumption.

How to Choose the Correct Answer in JLPT Questions

When both ことか and だろう seem possible, follow these steps.

Step 1: Identify the Main Purpose of the Sentence

Ask yourself whether the sentence is emphasizing emotion or expressing a judgment.

• Strong emotion or intensity: ことか
• Prediction, inference, or judgment: だろう

Step 2: Look for Evidence in the Surrounding Context

Expressions such as the following often provide a reason for conjecture:

• dark clouds are approaching
• someone has practiced every day
• no reply has arrived yet
• the train has already left

When the sentence uses evidence to reach a conclusion, だろう is often the better choice.

Step 3: Notice Words Such as どれほど and どんなに

Words such as どれほど, どんなに, and 何度 often appear with ことか.

However, this is not an absolute rule.

Both of the following sentences are grammatically possible.

「どれほど大変だったことか。」
How difficult it was.

「どれほど大変だっただろう。」
How difficult it must have been.

The final decision depends on whether the speaker is expressing emotion directly or imagining a situation.

Common Mistakes

Mistake 1: Treating ことか as a Normal Question

「どれほど苦労したことか。」
How much hardship I experienced.

This is not a request for information.

It strongly emphasizes that the experience was extremely difficult.

Mistake 2: Assuming That だろう Is Only Used for the Future

だろう can express conjecture about the present, past, or future.

• Present conjecture: 「彼は今ごろ家にいるだろう。」
  He is probably at home by now.

• Past conjecture: 「突然の知らせを聞いて、驚いただろう。」
  The person was probably surprised to hear the sudden news.

• Future conjecture: 「明日は気温が上がるだろう。」
  The temperature will probably rise tomorrow.

Mistake 3: Looking at Only One Keyword

The presence of どれほど does not automatically mean that the answer is ことか.

The surrounding context matters more than a single word.

One More Expression to Learn: ことだろう

Japanese also has the expression ことだろう (koto darō).

It combines emotional emphasis with conjecture.

「家族はどれほど心配したことだろう。」
How worried the family must have been.

The speaker may not know exactly how the family felt.

However, based on the circumstances, the speaker imagines that their worry was extremely intense.

Additional Examples

「海外で一人暮らしを始めた子どもを見送る親は、どれほど寂しいことだろう。」
How lonely parents must feel when they see off a child who is beginning to live alone overseas.

「長年努力してきた選手にとって、優勝はどれほどうれしいことだろう。」
How joyful winning the championship must be for an athlete who has worked hard for many years.

Final Summary: Look for Emotion, Probability, or Both

The difference between ことか and だろう cannot always be understood by looking at the final words alone.

You need to identify what the writer is trying to communicate.

• ことか: expresses the strength of an emotion or experience
• だろう: expresses a conjecture based on circumstances or evidence
• ことだろう: combines emotional intensity with conjecture

In JLPT reading questions, understanding individual words is not always enough.

You also need to recognize whether the writer is expressing emotion, making a judgment, or imagining someone else's situation.

This is the skill that helps you choose between two answers that both seem grammatically possible.

Related grammar guide: JLPT Inference and Judgment Expressions

RJT（Rapid Japanese Training）で、文脈を見抜く力を鍛える

Build your reading skills through short daily practice sessions and become more confident when choosing the best answer on the JLPT.