Difference between 〜たことになる and 〜ることになる when both are describing a past result


>   1.
> 都立校とは思えない充実した設備を誇るうえ、数年前に創立されたばかりのため内外装も損傷がほとんどない。もちろん旧被災地の高校らしく、地下シェルターも最新のものが設えられている。
>

>
> そのためか入試倍率は低くなく、「家が近いから」だけの理由で受験を決めた士道は、少々苦労をす **ることになった** のだが。(Date a live,
> novel1)
>
>   2.
> 「さて、二人とも。今日来てもらったのは他でもない。ーー宇宙空間を漂っていた先の精霊が<ラタトスク>の手に落ちたことで、彼らの元には累計一〇体もの精霊が集まっ
> **たことになった** 」(Date a live, novel16)
>

The context for 1 is that the text is introducing the background of the school
and the protagonist, 士道, who is a high school student. The sentence in
question is a flashback of how he worked hard and passed the school entrance
examination.

I think I understand the overall meanings of the two quoted texts. But I’m
wondering, since “苦労” and “集まる” are both past events, why don’t we use
苦労したことになった in the first sentence (but we do use 集まったことになった)? Generally, what
would be the difference between 〜たことになる and 〜ることになる when both are describing a
past result?




`[V dictionary form]-ことになる` expresses that something is decided upon. In your
example, the moment he decided to take the exam, his destiny was set so he
**would** have a hard time. Since this happened in the past, the final verb なる
is used in the past tense. However, 苦労する remains unchanged. The reason for the
present tense here is, as explained in other answers, this event hadn’t
happened by the time he decided to take the exam.

`[V た-form]-ことになる` is harder to explain. It is used when something stated
earlier makes it true that something has already completed. In your case, a
particular spirit fell into the enemy’s hands, and **that means** that they
have collected ten spirits in total. The reason 集まった is used in the past tense
is, again as explained in other answers, this event has already happened. The
reference point of time could either be the present or a little earlier when
the last spirit fell into the enemy’s hands.

As for the tense of the final verb なる in the second sentence, he could have
said 集まったことになる, as well. It even sounds more natural to me. But that’s off the
topic.

