〜ことだから

About

The ～ことだから pattern is used to express that, due to the speaker's beliefs/opinions regarding a given subject, "surely X will be the case/surely Y is true".

～ことだから may be attached to a noun, verb, or な-adjective, and mainly attaches to nouns that refer to a person, or a group of people.

E.g. あの二人ふたりのことだから、私わたしたちが付ついていなくても大丈夫だいじょうぶだろう。 "It's those two, so they should be okay even if we don't go with them."

E.g. マイクのことだから、昨日きのう怒おこられたことはもう忘わすれているよ。 "It's Mike, so I'm sure he's already forgotten (they) got mad at him yesterday."

When used in combination with a verb, ～ことだから is used to indicate that said action is unique or remarkable in some way ("and therefore...").

E.g. 久ひさしぶりにみんなで集あつまっていることだから、写真しゃしんをいっぱい撮とろう！ "This is the first time in a while that everyone is gathered together, so let's take a lot of photos!"

～ことだから may also end a sentence. In this case, the final から is dropped, and the "so..." is left implied.

E.g. 旅行好りょこうずきなボブさんのことだ。きっとこの冬ふゆもどこかに旅行りょこうに行いっているよ。 "Bob loves traveling. I'm sure he's going somewhere this winter."

～ことだから is most commonly used in casual or polite spoken scenarios.

Summary

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  The ～ことだから pattern is used to express "surely X will be the case/surely Y is true"

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  ～ことだから may be attached to a noun, verb, or な-adjective

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  ～ことだから is most commonly used in casual or polite spoken scenarios