How do ことになる and ようになる differ? (When ことになる cannot = "It has been decided that")


I understand that ことになる has the meaning "It has been decided that" and can be
used to point to a ritual or habit of a society like taking off your shoes.

However, I don't get what it means when it is like this:

> あすで１週間雨が降りつづくことになる。

How is that different from say,

> あすで１週間雨が降りつづくようになる。




First, I must clearly state that " **it has been decided that ~~** " is a
highly overrated translation of 「～～ことになる」 among J-learners. Truth is that that
is not what it means even half the time.

「あすで１週間雨{しゅうかんあめ}が降{ふ}りつづくことになる。」 means " **It will have ended up raining a
whole week**." It has already been raining for 6 straight days now (at the
time of utterance) and the weather forecast says that it will rain again
tomorrow.

A more natural or less stiff way to say this would be
「もし明日{あす}も雨だったら、（これで）一週間雨が降り続{つづ}いたことになる。」, but virtually all native speakers
would understand 「あすで１週間雨が降りつづくことになる。」.

Here, 「ことになる」 means " **to end up (in a certain way)** " and that is already a
very common usage of the expression.

Your second sentence 「あすで１週間雨が降りつづく **よう** になる。」, however, makes little sense,
I am afraid.

「ことになる」 refers to (and focuses on) the final result whether it happened
naturally or someone made it happen on purpose.

「ようになる」 focuses on the change from one situation to another. "It was like A
before but it is like B now." e.g. 「去年{きょねん}までは泳{およ}げなかったが、今年{ことし}からは泳げる
**ようになった** 。」 In this example sentence, you cannot replace the 「ようになった」part
with a 「ことになった」 because it talks about a **change** in one's swimming ability.




First, I must clearly state that " **it has been decided that ~~** " is a
highly overrated translation of 「～～ことになる」 among J-learners. Truth is that that
is not what it means even half the time.

「あすで１週間雨{しゅうかんあめ}が降{ふ}りつづくことになる。」 means " **It will have ended up raining a
whole week**." It has already been raining for 6 straight days now (at the
time of utterance) and the weather forecast says that it will rain again
tomorrow.

A more natural or less stiff way to say this would be
「もし明日{あす}も雨だったら、（これで）一週間雨が降り続{つづ}いたことになる。」, but virtually all native speakers
would understand 「あすで１週間雨が降りつづくことになる。」.

Here, 「ことになる」 means " **to end up (in a certain way)** " and that is already a
very common usage of the expression.

Your second sentence 「あすで１週間雨が降りつづく **よう** になる。」, however, makes little sense,
I am afraid.

「ことになる」 refers to (and focuses on) the final result whether it happened
naturally or someone made it happen on purpose.

「ようになる」 focuses on the change from one situation to another. "It was like A
before but it is like B now." e.g. 「去年{きょねん}までは泳{およ}げなかったが、今年{ことし}からは泳げる
**ようになった** 。」 In this example sentence, you cannot replace the 「ようになった」part
with a 「ことになった」 because it talks about a **change** in one's swimming ability.

