～ことだから — Knowing Them, Surely…
Used to make a confident prediction based on the well-known character or nature of a specific person (or group). It means 'because it's ~ (whom we know so well), surely…' The speaker draws a conclusion precisely because of shared knowledge about that person's typical behavior.
Structure
[Noun (person) + の] + ことだから
Examples
真面目な彼のことだから、きっと約束を守るでしょう。
Majime na kare no koto da kara, kitto yakusoku o mamoru deshou.
Being the diligent person he is, he'll surely keep his promise.
彼のことだから + prediction based on his known character
田中さんのことだから、もう準備は終わっているはずだ。
Tanaka-san no koto da kara, mou junbi wa owatte iru hazu da.
Knowing Tanaka, the preparations are surely already done.
田中さんのことだから draws a confident guess from Tanaka's usual reliability
あの子のことだから、また遅刻するに違いない。
Ano ko no koto da kara, mata chikoku suru ni chigainai.
Knowing that kid, he's bound to be late again.
ことだから + negative prediction based on the child's habitual behavior
• The noun is almost always a specific person (or organization) the speaker knows well. • Usually followed by a guess/prediction (でしょう, はず, に違いない, きっと). • The prediction is justified by that person's known personality or habits.
Commonly confused with
The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～ことだから is tested against.
vs ～のことだ
Plain のことだ just means 'about ~'; ことだから specifically uses knowledge of someone's character to justify a prediction.
vs ～だから
だから gives a general reason; ことだから adds the nuance 'because it's this particular person we know, we can predict their behavior.'
More cause & effect
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