ことだから details
knowing X as we do; since it's X — a prediction grounded in a known party's character
あの人のことだから、また忘れてるんじゃない？
ano hito no koto da kara, mata wasureteru n ja nai?
Knowing him, he's probably forgotten again, don't you think? casual
時間に厳しい田中さんのことだから、もう着いているでしょう。
jikan ni kibishii tanaka-san no koto da kara, mou tsuite iru deshou.
Tanaka being the stickler for time that he is, he'll have arrived already. neutral
- polite
Examples
knowing X as we do — the speaker predicts what X will have done or will do, from what both parties know X to be like
あの人のことだから、また忘れてるんじゃない？
ano hito no koto da kara, mata wasureteru n ja nai?
Knowing him, he's probably forgotten again, don't you think? casual
時間に厳しい田中さんのことだから、もう着いているでしょう。
jikan ni kibishii tanaka-san no koto da kara, mou tsuite iru deshou.
Tanaka being the stickler for time that he is, he'll have arrived already. neutral
几帳面な部長のことですから、資料はもうご用意なさっているはずです。
kichoumen na buchou no koto desu kara, shiryou wa mou go youi nasatte iru hazu desu.
The director being as meticulous as he is, he will surely have the materials ready. formal
Formation
{N}のことだからattaches to noun
Notes
conjecture (きっと〜だろう／〜はずだ). A plain fact after it (×彼の
ことだから遅れました) is not this pattern.
Related patterns
Similar
からすると reasons from evidence in front of the speaker or a fact just stated (この散らかりようからすると); のことだから reasons from a standing trait of a person both speaker and hearer already know, with nothing to point at — swap からすると in and the sentence starts demanding evidence the hearer can inspect.
Lookalikes
One nominal head apart and constantly swapped, but they run in opposite directions in time — ことだから grounds a PREDICTION in a known party's character (彼のことだから、また遅れるだろう) and its main clause is conjectural, while ものだから excuses something ALREADY done by a cause the speaker did not control (寝坊したものだから、遅れました); ×彼のことだから遅れました excuses nothing.
から's noun row is {N}だから and this adds only のこと, but that のこと forces the noun to be a person (or a group treated as one) the hearer also knows and forces a guess in the main clause — 雨だから中止した is ordinary where ×雨のことだから中止した is not.