Koto and Mono

I've come across some cases of koto and mono that I want some more info on.
I know that these are nominalizers, and I know that koto is for intangible things while mono is for tangible things.

1. 子供のとき、この公園でよくサッカーをしたものだ。 Tangible
When I was a kid, this park was a great place to play soccer.

2. 来年、日本に留学することになりました。 Intangible
Next year, (I know the words but translation in English I'm not so sure)

By the way, for #2 , I'm quite confused as to why する is added to 留学 if こと was just going to be added to it to change it back into a noun. My dictionary says 留学 means "Studying abroad" which is a noun and suru verb, so why was する added? Furthermore, I just caught this one. The sentence starts off with "Next year", but it ends with a verb in past tense.

Anyway! Sorry for getting off topic.
Sometimes, I see sentences ending with こと or もの.

3. 盗むのは悪いことだ。
It is bad to steal.

Why was こと added in #3 ? Is it an explanatory tone? I know that もの can be used for explanations, but can こと do the same? I'm also thinking that maybe adding こと to it gives more of a base as to what stealing is. Like, adding こと is saying that the concept of stealing entirely is bad. Without こと, you may not be as clear to this. Am I on the right track?

Thanks!

All those three expressions are set phrases.

1)
"The past form of a verb + ものだ" expresses a past habit, i.e., it means "used to ~".

3 〔過去に度々したこと〕
若いころはよく山登りをしたものだ
I used to go mountain climbing frequently when I was young.

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ものの英語・英訳 - 和英辞書 - goo辞書

2)
"The present form of a verb + ことになる" expresses "a plan will be settled(= the plan is not settled yet now)", thus, "the present form of a verb + ことになった" shows "the plan is/has been settled". In your example sentence, the plan that the speaker will go to Japan next year is already settled, therefore the past form is used there.

1 〔動詞について「…すること」の形で，行動，予定〕
行くことにした
I've decided to go.
彼は9時に来ることになっている
He is due here at nine.
駅で落ち合うことになっている
We are to meet at the station.
そういう本を読むことはめったにない
I seldom read books like that.
日曜日には母に手紙を書くことにしている
I make it a rule to write to my mother on Sundays.

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こと【事】の英語・英訳 - 和英辞書 - goo辞書

You need to use a verb before this meaning of ことになる, that's why する is necessary there. (In other words, 留学することになった and 留学になった have completely different meanings.)

3)
See the following thread.
https://jref.com/forum/learning-j...6%82%BE-48229/

Thank youuuuu now that you say that they're expressions, I think I recall seeing them somewhere in my Japanese notebooks.