Japanese Grammar

How to Express Various Levels of Certainty in Japanese

Probably: だろう and でしょう

You have started a new section called “modality.” This is a grammatical term which you can simply consider to mean “the speaker’s feelings.” For example, you will learn how to express certainty, judgments, volition, commands, requests, obligation, etc. In this lesson, you will tackle how to express various levels of certainty.

Explanation for the Usages of だろう, かもしれない, はずだ, and にちがいない

Table of Contents
だろう / でしょう: Probably
かもしれない: May
はずだ and にちがいない: Supposed to Be, Should, and Must

In English, you can express a level of certainty using “must,” “should,” and “can,” e.g. Japanese must be easy to learn. In Japanese, we express it by conjugating verbs instead. Let’s check out how they work.

だろう / でしょう: Probably

明日あした あめ る(だろう / でしょう)
Topic Subject Verb + だろう
As for tomorrow, it will probably rain.

だろう can give a nuance of “probably” to sentences. However, this sounds very masculine and thus we recommend you use the polite form, でしょう, if you are female. You can directly connect them with any element such as verbs, adjectives, and nouns. In practice, they are often used with adverbs in formal situations.

明後日あさってれ(だろう / でしょう)。
As for the day after tomorrow, [it] will probably be sunny.
今年ことしなつはおそらくすずしい(だろう / でしょう)。
As for this summer, [it] will probably be cool.
明日あしたにはげんになる(だろう / でしょう)。
By tomorrow, [I] will probably be alright.
ゆきらない(だろう / でしょう)。
[It] won’t probably snow.

The explanatory のだ can be used with だろう. If you have a doubt about the usage, please check the previous lesson: Explanatory のだ (んだ).

明後日あさってれ(なの / なん)(だろう / でしょう)。
今年ことしなつはおそらくすずしい( / )(だろう / でしょう)。
明日あしたにはげんになる( / )(だろう / でしょう)。
ゆきらない( / )(だろう / でしょう)。

Note: だろう has functions to confirm something and blame someone. You will learn them in other lessons.

Practical Alternative: おも

Since だろう is preferred in formal situations, you can alternatively use 思う to express your certainty in everyday life. They have almost the same nuance. When you use 思う, your speech sounds like your personal opinion. If you need to speak objectively, the passive form is more suitable than the plain form.

明後日あさってれだと(おもう / おもいます)。
As for the day after tomorrow, [I] think it will be sunny.
今年ことしなつすずしいとおもわれ(る / ます)。
As for this summer, it is thought that it will be cool.


かもしれない: May

[わたしは] ほん すかもしれ(ない / ません)
[Topic / Subject] Destination Verb + かもしれない
[I] may move to Japan.

かもしれない expresses there are some possibility and can roughly be translated as “may” in English. Just like だろう, you can connect かもしれない with any element without conjugation.

明日あしたゆきかもしれ(ない / ません)。
As for tomorrow, [it] may snow.
漢字かんじむずかしいかもしれ(ない / ません)。
Kanji may be difficult.
あまりょうべないかもしれ(ない / ません)。
As for sweet meals, [I] may not eat [it].
パーティーにはかないかもしれ(ない / ません)。
As for the party, [I] may not to go.

The explanatory のだ can also appear with it. For the sake of simplicity of pronunciation, the form: んだ is not used regardless of the formality.

明日あしたゆきなのかもしれ(ない / ません)。
かんむずかしいかもしれ(ない / ません)。
あまりょうべないかもしれ(ない / ません)。
パーティーにはかないかもしれ(ない / ません)。

In casual tone, people sometimes abbreviate かもしれない like the following. The meaning remains the same.


はずだ and にちがいない: Supposed to Be, Should, and Must

なかさんは いま 会社かいしゃ いるはず(だ / です)
Topic / Subject Adverbial Noun Location of Existence Verb + はずだ
Tanaka-san should be in the office now.

はずだ expresses high certainty and can be translated as “supposed to be,” and “should.” When you connect nouns and na-adjectives, you need to attach の to nouns and な to na-adjectives respectively. This implies that you reach certainty through an objective thinking.

今日きょう会社かいしゃやすはず(だ / です)。
Today, our office should be closed.
書館しょかんしずはず(だ / です)。
Libraries should be calm.
これはべられるはず(だ / です)。
This should be eatable.

When you make negative sentences, there are two sentence patterns. The first is to conjugate a part of speech connected with はずだ. The second is to conjugate はずだ. Although the meanings are the same, the second one sounds stronger.

これはべられないはず(だ / です)。
This shouldn’t be eatable.
これはべられるはずがない / ありません)。
=> Stronger than the above
なかさんは いま 会社かいしゃ いるにちがい(ない / ありません)
Topic / Subject Adverbial Noun Location of Existence Verb + にちがいない
Tanaka-san must be in the office now.

に違いない also expresses high certainty just like はずだ, but implies you reach certainty through a subjective thinking. Thus, に違いない can be used to express instinct judgments. There is no conjugation. You can directly connect this with any element. This sounds a little formal and is preferred in writing.

田中たなかさんは親切しんせつちがい(ない / ありません)。
Tanaka-san must be kind.
すずさんはせいちがい(ない / ありません)。
Suzuki-san must be a politician.
こくするにちがい(ない / ありません)。
[I] must be late.
先生せんせい今日きょう学校がっこうないにちがい(ない / ありません)。
It must be that the teacher won’t come to school today.

Since に違いない sounds formal, you can alternatively use 思う with strong adverbs to express your subjective opinions in conversation.

なかさんは絶対親切ぜったいしんせつだと(おもう / おもいます)。
[I] think Tanaka-san is definitely kind.
すずさんはちがいなくせいだと(おもう / おもいます)。
Without doubt, [I] think Suzuki-san is a politician.


  1. だろう can give the nuance of “probably” to sentences, but sounds formal.
  2. かもしれない expresses there is some possibility like “may.”
  3. はずだ expresses high certainty and implies objective opinions.
  4. に違いない expresses high certainty and implies subjective opinions, but sounds formal.
  5. You can substitute 思う with adverbs for the above expressions.

If you look at the formality, only かもしれない and はずだ are suitable for conversation. Therefore, we can say that No. 5 in the summary is the key to natural speech, though you should not frequently use 思う in writing. The conjugation is very simple. Let’s master the usages here. Next you will learn how to express judgement.

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