〜ことになる
It has been decided that… / it turns out that…machine translation
FormationVerb dictionary form / ない-form + ことになる (rules: ことになっている)
ことになる expresses a decision that comes FROM OUTSIDE — a company, a school, circumstances — rather than your own choice: 来月大阪へ引っ越すことになりました (it's been decided that I'm moving to Osaka next month — for example, because my company is transferring me). The form ことになりました is how you announce the outcome of such a decision.
Interestingly, Japanese people also use ことになりました for their OWN decisions when they want to sound modest and avoid showing off — the classic case is announcing a marriage: 結婚することになりました. The literal meaning, "things have come to be this way", makes the sentence sound much gentler than 結婚することにしました (I decided to get married).
The form 〜ことになっている refers to rules and regulations currently in effect: 学校では、携帯電話を使ってはいけないことになっています (the school has a rule against using mobile phones). When you need to cite a rule without saying who made it, this is the standard pattern. Remember the contrast: ことにする — you decide; ことになる — it gets decided; exactly the する (active) vs. なる (happens on its own) relationship you already know.
Examples
来月大阪へ引っ越すことになりました。
It has been decided that I'm moving to Osaka next month.
会議は金曜日に行われることになりました。
It was decided that the meeting will be held on Friday.
学校では、教室で携帯電話を使ってはいけないことになっています。
At school there's a rule against using mobile phones in the classroom.