ことにする・
ことにする is "I decide to": the choice is yours, and you say so. ことになる is "it's been decided", and it doesn't say who chose. With ている on the end the decision still stands: ことにしている is your own rule, ことになっている is the rule.
you decide
I've decided to leave the company next month.
it is decided
It's been decided to put the meeting off until next week.
the rule
You're not supposed to use cell phones during exams.
How to form it
Verb (dictionary or ない-form) + ことにする／ことになる
行くことにした, 行かないことになった. It is こと, not の. Past, polite and ている all go on the end: ことにしました, ことになっています.
ことにする
You decide.
I've decided to get up at six from tomorrow.
ことになる
It is decided.
It's been decided that we're getting up at six tomorrow.
ことにしている
You decided, and you stick to it: your own rule.
I make it a rule to get up at six every morning.
ことになっている
It was decided, and it stands: the rule.
In this dorm, we're supposed to get up at six every morning.
ことにする: you decide
The rain's heavy, so I'll go home by car today.
Deciding as you speak: "I'll …".
The rain's heavy, so I've decided to go home by car today.
Already decided, even a minute ago: "I've decided to".
I'm tired today, so I won't make dinner.
Deciding not to.
ことになる: it is decided
Often someone else decided: your company, a meeting, everyone together. Sometimes nobody did, and things just turned out that way: English "ended up".
I've been put in charge of reception.
We talked it over and agreed to meet again next week.
The trains stopped, and I ended up staying near the station that night.
Who decided?
I've decided to study abroad next year.
Your own choice.
I'm going to be studying abroad next year.
It has been settled. Perhaps your company is sending you.
When someone else made the decision, ことにしました is wrong: it says the choice was yours.
Your own news: 結婚することになりました
People also announce their own decisions with ことになりました: a wedding, a move, leaving a job. It reports how things have turned out, not what you chose, so it sounds modest.
We're getting married.
The announcement. They decided it themselves.
We've decided to get married.
Also said. It tells of making up your minds.
I'm moving to Tokyo next month.
ことにしている: your own rule
English says "I make it a rule to", or just "I always" and "I never".
I make it a rule to take a walk after dinner every day.
At home, I never use my phone during meals.
ことになっている: the rule
A rule, a custom or an arrangement that is in place. English says "supposed to".
In Japan, you're supposed to take your shoes off when you go into a house.
A custom.
I'm supposed to meet Tanaka at three tomorrow.
An arrangement.
Close neighbours
An effort, not a rule: you try to, and some days you slip.
I try not to drink coffee late at night.
ようになるN4
A change that has come about, not a decision: what you can do now, or have started doing.
Since coming to Japan, I can now eat natto too.
予定だN4
What is on the schedule, and no more. A rule or a custom takes ことになっている, not 予定.
This train is scheduled to arrive at 9:10.
たことにするN2
A past-tense verb before こと makes it pretending: you act as if something happened, or didn't.
I'll pretend I didn't hear that.
Check yourself
1Your company is moving you to its Nagoya office from April. You didn't ask to go. How do you tell your neighbor?
四月から
Choose an answer to see why.
2What does this mean?
私は
Choose an answer to see why.
3A classmate tells the class this. Which is true?
来月、
Choose an answer to see why.
More detail
In conversation
The い of ている usually drops: ことにしてる, ことになってる.
I don't work on Sundays. It's a rule of mine.
We're supposed to meet at the station at five tomorrow.
"That comes to"
ことになる also works out what something amounts to: "that means", "that comes to". Nothing was decided.
If you spend 500 yen a day, that comes to 15,000 yen a month.
More sentences
After talking it over with my family, I decided to work in Osaka from next year.
After a lot of thought, I've decided to apply this time.
Because of the typhoon, the game has been put off until the following week.
We drew lots, and I ended up being the one to present for the group.
I make it a rule to go over the week's spending every Sunday.
In this dorm, you're not supposed to use the washing machines after 10 p.m.
In this lab, the last person to leave is supposed to close the windows.
At work we're supposed to share our schedules by nine each morning, but I make it a rule to enter mine the night before.