What it means
ことだから expresses a reason based on the known nature, character, or circumstances of someone or something. It is often used to make a confident prediction or inference: given that it is X, the result is naturally expected. The most common pattern is a noun followed by のことだから, especially when referring to a person whose usual behavior is known.
Usage note
ことだから is not usually a neutral way to state an ordinary cause. It often carries the speaker’s personal knowledge or expectation and is especially common with people’s names or pronouns, as in 彼のことだから. Do not confuse it with ことだから used in expressions such as いつものことだから, where it simply refers to something usual.
How to use it
彼のことだから、きっと成功します。
Knowing him, he will surely succeed.
彼のことだから、きっと成功する。
Knowing him, he will surely succeed.
Example sentences
部長のことだから、必要な資料をすべて確認してから、申請を承認し、関係者に連絡するだろう。
Knowing the manager, the manager will probably check all the necessary documents, then approve the application and contact the relevant people.
姉のことだから、旅行の準備を早く終えて、もう空港へ向かっているだろう。
Given that she is my older sister, she has probably already finished preparing for the trip and is heading to the airport.