1The most basic noun use — refers to a 'matter', 'thing', or 'affair' in the abstract sense (not a physical object). Common in fixed phrases like そのこと (that matter), いいこと (a good thing), 悪いこと (a bad thing). Almost always written in kana in modern writing.
そのことは私もよく知っています。
I'm well aware of that matter too.
今日はいいことがあった。
Something good happened today.
悪いことをしたら、ちゃんと謝りなさい。
If you do something bad, apologize properly.
そんな小さなことで悩むな。
Don't worry over such a small matter.
2An event, incident, or — especially in phrases like 大変なことになった (this has turned into a serious situation) — something gone wrong or a crisis. Refers to a happening, often with implied weight.
電車が止まって、大変なことになった。
The trains stopped — this has turned into a real mess.
あの事件のことは、まだはっきり覚えている。
I still remember that incident clearly.
もしものことがあったら連絡してください。
If anything happens (any emergency), please contact me.
3Circumstances or situation surrounding a topic — 「経済のこと」(matters concerning the economy), 「天気のこと」(matters of weather). Used to refer broadly to the state of affairs around something.
最近、経済のことが心配でならない。
Lately, I just can't stop worrying about the economy.
天気のことを考えると、明日の遠足は無理かもしれない。
Considering the weather, tomorrow's outing might not be possible.
4Work, business, or matters one is responsible for. Used in phrases like 家のこと (housework / domestic matters), 仕事のこと (work matters).
家のことは私に任せて、安心して出張に行ってきて。
Leave things at home to me — go on your business trip with peace of mind.
仕事のことで頭がいっぱいで、休みの日も休めない。
My head is so full of work that I can't even relax on days off.
5The 「Xのこと」 pattern — marks a person or topic as the object of a thought, feeling, or judgment. Hugely common: 彼のことが好き (I like him), 子供のことが心配 (I'm worried about my child), あの人のことだから (knowing that person…). Japanese often prefers 「のこと」 over a bare object when the object is a person (彼のことが好き sounds more natural than 彼が好き).
私は彼のことが本当に好きです。
I really do like him.
親はいつも子供のことを心配しているものだ。
Parents are always worried about their children.
あの人のことだから、きっと時間通りに来るよ。
Knowing that person, he'll surely arrive on time.
君のことを忘れたことは一度もない。
Not for a single moment have I forgotten about you.
6Nominalizing suffix — attaches to the plain form of a verb or adjective to turn the clause into a noun phrase. 勉強することは楽しい (Studying is fun), 早く寝ることが大切だ (Sleeping early is important). One of the most fundamental Japanese grammar patterns; encountered from beginner level.
日本語を勉強することはとても楽しい。
Studying Japanese is really fun.
毎日運動することが健康の秘訣だ。
Exercising every day is the secret to good health.
彼に会えなかったことが残念でならない。
I deeply regret not being able to meet him.
若いうちにいろいろなことを経験するべきだ。
You should experience all kinds of things while you're young.
7Used in 〜ことにする / 〜ことにしている to mean (1) decide to do something (volitional decision: 'I've decided to ...'), or (2) treat/pretend that something is so (even if it isn't really). Use (1) is one of the most common ways to express a personal decision in everyday Japanese.
来年からジョギングを始めることにした。
I've decided to start jogging from next year.
毎朝六時に起きることにしている。
I make it a habit to wake up at six every morning.
今日のことは、聞かなかったことにしておこう。
Let's just pretend I didn't hear about today.
8In the form 「AことB」 — indicates that A is an alias, pen name, or former name and B is the real name. A formal, somewhat old-fashioned usage seen in newspaper articles and official notices when introducing stage names or maiden names.
国民的歌手の美空ひばりこと加藤和枝が、十八歳でデビューした。
The beloved singer Misora Hibari — real name Kato Kazue — made her debut at eighteen.
ねずみ小僧こと次郎吉は、江戸時代の有名な義賊だ。
Nezumi-kozo — real name Jirokichi — was a famous outlaw-hero of the Edo period.
9Used in the construction 〜ことはない to mean 'there's no need to ...' or 'you don't have to ...'. Often used to reassure someone who is worrying or trying too hard.
そんなに急ぐことはないよ、ゆっくり食べて。
There's no need to rush — eat slowly.
失敗しても、そこまで落ち込むことはない。
Even if you fail, there's no need to be that depressed.
迷っているなら、無理して行くことはないと思うよ。
If you're unsure, I don't think you need to force yourself to go.
10Used in 〜ことだ to give advice or a recommendation — 'you should ...' or 'the important thing is to ...'. Can sound slightly preachy or top-down, so used carefully with seniors.
上手になりたいなら、毎日練習することだ。
If you want to get good, you should practice every day.
健康のためには、よく寝て、よく食べることだ。
For your health, the key is to sleep well and eat well.