ことだから

のことだから bases a guess on what you know of someone: "knowing Tanaka", "Tanaka being Tanaka". It follows a person's name, a pronoun or a word for a person, joined with の, and the second half says what you expect of them.

The guess can be good or bad, and the second half usually carries きっと, だろう, にちがいない違いない or はずだ. You need to know the person already; it makes no sense about someone you have just met. A description in front of the name – 真面目な, 優しい – often spells out the trait the guess rests on.

Leaving out の is the usual slip: ×田中さんことだから.

After a clause

ことだから can also follow a plain-form verb or a な-adjective. The reason is then the nature of the event rather than of a person: "since it's something a child did", "since it was so sudden".

ことだから, ことだ and ことから

The three look alike. ことだ gives advice ("you should"), and ことから names the fact a conclusion was drawn from ("from the fact that"). Only ことだから predicts from someone's character.