Grammar Info
N3 Lesson 3: 11/23

事(こと)になる
It will end up being, It is the result of, It has been decided that

The "something has been decided" meaning only applies to verbs!

Structure
Casual
Formal
Verb + ことになる
[い]Adjective + ことになる
[な]Adjective + + ことになる
Verb + ことになります
[い]Adjective + ことになります
[な]Adjective + + ことになります
Details
Register Standard
使用域 一般
Information
ことになる is an expression that combines the nominalizing use of こと (creating a noun-phrase), with the 格助詞(かくじょし) (case marking particle) , and the う - Verb なる ‘to become’. This expression can be translated as ‘to become (A)’, but has the nuance that someone/thing caused (A) to become that way. Due to this, it may also be translated as ‘it has been decided that (A)’, or ‘it will end up being (A)’.
ことになる is primarily used with verbs and い-Adjectives in their 連体形(れんたいけい) (attributive form). It may also be used with な-Adjectives followed by な.
  • なんでも後回(あとまわ)しているあとから苦労(くろう)することになる
    If you wait until it is too late, you are going to end up regretting it.
  • そんなことたらとんでもないことになるよ。
    If you do something like that, it is going to become a huge deal.
  • 出来(でき)るだけ早(はや)く治療(ちりょう)ないと、後(あと)から大変(たいへん)なことになる
    If you don't treat it soon, it is going to end up being a bigger issue.
The ‘it has been decided that (A)’ meaning only applies to verbs, as only verbs will be enacted by someone/thing.
Fun Fact
This structure is a good example of why なる is often viewed as the intransitive form of する. する conveys that someone is performing an action (transitive), while なる implies that something is the result of an action (intransitive). This is why the (A) phrase in ことになる is usually determined to have been decided/performed by someone else, while still focusing on exactly ‘what’ (A) became.
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明日(あした)雨(あめ)ならば、1週間(しゅうかん)降(ふ)り続(つづ)くことになる
If it rains tomorrow, it will end up being a week straight of rain.
Notice that when ことになる means 'end up/ it is the result of' it is often preceded by conditional clause. Also, it often expresses a negative situation.
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震災(しんさい)のため、アメリカに帰(かえ)らなければならないことになりました
It was decided that I must return to America because of the earthquake.
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インフルエンザにかかってしまったので、今日(きょう)の大会(たいかい)が出(で)れないことになった
Since I got the flu I ended up being unable to attend today’s tournament.
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Books
Tobira
Page 172
[DBJG] A Dictionary of Basic Japanese Grammar
Page 202
Tae Kim's Japanese Grammar Guide
Page 131
[AIAIJ] An Integrated Approach to Intermediate Japanese
Page 15