事(こと)だから

ことだから is a set expression used for indicating that the speaker is fairly certain about (B), based specifically on their experience with (A). This is often translated as 'it is exactly because (A) that (B)', or 'precisely because it is (A), (B)'. ことだから is a combination of the noun こと 'thing', だ, and から. The literal meaning can be interpreted as 'from that it's (A), also (B)'. This just implies that due to (A) being a predetermined fact, (B) too can be assumed to be true.
When making assumptions, ことだから will often be seen following verbs in their dictionary forms, or nouns followed by の.

• あの人(ひと)がすることだから、どうせ人(ひと)を騙(だま)して儲(もう)けているに違(ちが)いない。
  Precisely because it is something that person does, there is no doubt that he is making money scamming people.
• いつも遅(おく)れてくる田中(たなか)くんのことだから、今日(きょう)も遅(おく)れてくるだろう。
  It is exactly because Tanaka-kun always arrives late, that he will probably arrive late today.

Alternatively, ことだから may appear following verbs in any of their standard forms when the situation of (A) is considered as a good opportunity to also do (B), according to the speaker.

• お父(とう)さんとお母(かあ)さんが珍(めずら)しくうちに来(き)ていることだから、久(ひさ)しぶりにみんなで映画(えいが)でも見(み)よう。
  Dad, mom, considering that you guys are here, which is a rare occasion, why don't we watch a movie together for the first time in a while.
• 撮影(さつえい)が無事(ぶじ)終(お)わったことだから、打(う)ち上(あ)げでもしましょう！
  Considering that we have finished filming, it is a good opportunity for us to have a party!

Lastly, ことだから may simply imply (strongly) that (A) is the reason for (B). In these cases, either verbs in their standard forms or nouns followed by の may precede ことだから.

• お客様(きゃくさま)のプライバシーに関(かか)わることなので、これ以上(いじょう)詳(くわ)しいことは言(い)えません。
  Because this is something that has to do with the privacy of our customer, we can't give you further details.
• 部長(ぶちょう)のことだから、またミスをしたらクビにさせられると思(おも)う。
  It is exactly because it is my boss, he will probably fire me if I mess up again.

Despite having these three slightly different nuances, the speaker's intended meaning will usually be clear in the (B) part of the sentence. Due to this, ことだから may always be thought to simply mean that 'by the logic of (A), definitely (B)'.