決What you think and know
Deciding
~ことにする is your own choice. ~ことになる says it was decided, whether by others or by circumstances.
Who decided?
Same event, different feeling
I decided to move to Osaka next month.
My decision.
It's been decided I'll move to Osaka next month.
It came about, maybe by my company.
~ことにする
A decision you make
I've decided to walk every day.
I've decided not to drink any more.
Decide not to: negative before こと.
I make a point of reading the paper every morning.
A habit you've chosen.
I'll have the fish.
With a noun, it's just に + する.
~ことになる
It's been arranged
It's been decided I'll study abroad from April.
Even for your own decision, this sounds modest.
I've been told to go on a business trip next week.
It's a rule that you don't make calls in the library.
~ことになっている describes a rule or custom.
A compare
Humility
Even when it was your own choice, ~ことになりました sounds more modest, so people often use it for big news such as marriage or a new job. ~ことに決める is a stronger, more serious version of ~ことにする, and can't mean “I hereby decide”.
Check yourself
Who decided?
Your turn
します or なりました?
Say it yourself
Big news
Tell your coworkers your news: your company is sending you to Osaka next spring, and because of that you've decided to study Kansai dialect.
Automatic feedback on your own answers is coming to Premium.