事(こと)だからIt is exactly because

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.

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.

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.

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)'.