How to Say "Must Be" in Japanese

に違いない

ni chigainai

In Japanese, "must be" is expressed as に違いない (ni chigainai).

The phrase に違いない is used to express a strong belief that something is certainly true based on evidence.

What does it mean word for word?

Literally, the phrase means "cannot be different/wrong".

ni chigainai

In detective novels or mystery anime, characters often use に違いない to declare a culprit's identity during a climactic reveal.
The word 違う inside this phrase is so common that it is often used as a one-word reply meaning 'No' or 'That is not it' during a conversation.
If you use に違いない to express a personal opinion to a superior, it may sound like you are declaring an absolute truth that cannot be debated. This can come across as arrogant because you are essentially stating that your observation is the only possible reality.

彼女は病気に違いない。
She must be sick.
This sentence is useful for expressing the certainty that someone is sick using に違いない.
Tatoeba #87018 by bunbuku, translation #316692 by CK
彼の話は本当に違いない。
His story must be true.
This sentence is useful for expressing the certainty that a story must be true using に違いない.
Tatoeba #115746 by bunbuku, translation #287924 by CK
私の兄は病気に違いない。
My brother must be sick.
This sentence is useful for expressing the certainty that a brother is sick using に違いない.
Tatoeba #163833 by bunbuku, translation #250677 by CK
20 more sentences on Tatoeba

Hiyori and Yota find a mysterious gift on the table.

陽葵 Himari

これ、誰からのプレゼントかな？
Kore, dare kara no purezento kana?
I wonder who this present is from?
陽太 Yota

中身からして、お母さんに違いないよ。
Nakami kara shite, okaasan ni chigainai yo.
Judging by the contents, it must be from Mom.
陽葵 Himari

たしかに、お母さんが好きそうな色だね。
Tashikani, okaasan ga sukisou na iro dane.
True, it is a color Mom would like.
陽太 Yota

だから、お母さんに違いないと思う。
Dakara, okaasan ni chigainai to omou.
That's why I think it must be her.

Sources

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