こと

Thing (Intangible)

こと is a noun that means “thing (intangible)”. This thing can refer to ideas, experiences, things to do, etc; but not physical objects.

There is a strong tendency for こと to be followed by は or が

Examples

• することがない – Nothing to do.
• できることがない – Nothing (I) can do. 
• 一番好きなこと – (My) favorite thing
• 辛いことはいつか終わる – Difficult things (times) will end sometime.
• 小さいことでわかれた – Broke up over a small thing.
• 大きいことを見つけた✖ – I found a big thing (non physical). 
  
  

Verb Nominalization

The thing of Verb
Verb Dictionary Form + こと

A consequence of こと meaning “thing” is that it can be used to nominalize adjectives, verbs, and nouns, as well as sentences.
こと can be used with the dictionary form of verbs, just like の:

• ウエブサイトを作ることは大変だ – Making a website is tough.
• 一番楽しいことは、日本語を勉強することだ。The most fun thing is studying Japanese.

こと and の

With verbs, こと and の are almost the same thing. The key differences are:

• こと must be used in こと set phrases (ことができる、たことがある、ことにする, etc.)
• こと must be used before だ・です
• こと must be used if you are dealing with communication. 
• の should be used before senses, stopping words, and “linked” verbs. (wait for, help with, etc.)
• In most other cases they should be interchangeable.

Nuance wise (when either may be used):

• こと is a bit more abstract
• の is more immediate

More information/Source

Noun Nominalization

About Noun/Concept of Noun
Nounのこと

“How do you nominalize a noun?” I hear you thinking. The answer is by making it a slightly broader noun. こと changes the meaning of a noun to “about noun” or “Noun things”. It is very commonly used with people.

• 彼のことが好き – I like him things.→ I like about him (and things pertaining to him, like the way he…). 
• 日本のことが好き – I like Japan things.

Comparison:

• 彼女のことは忘れない – I won’t forget about her. (things she did, etc)
• 彼女は忘れない – I won’t forget her (that she existed) (OR she won’t forget). 

Time words

こと should be used with past and future (not present) time words for clarity, especially when not using は.

• 昨日のことはありがとう – Thank you (for the thing you did yesterday)
• 昨日はありがとう△ – Yesterday, thank you. (ok)
• 明日を忘れよう ✖- Let’s forget tomorrow. (that it exists?)
• 明日のことは忘れよう – Let’s forget about (that thing) tomorrow. 
• 来年のことは来年で考えよう – Let’s think about next year, next year. 

Sentence Nominalization

That (conjunction)
Sentence+ということ

こと can also be used to nominalize a sentence by using plain form sentence + ということ. This effectively makes the entire sentence into a giant compound noun. 
Be careful not to forget だ after nouns and な adjectives.

• 彼がすごいだということはわすれない – I’ll never forget that he is strong. 
• 夜がこんなにしずかだということは知らなかった – I didn’t know that night was this quiet.
• 先生はビールが好きだということは知っている – I know that the teacher likes beer.
• クマが肉食だということは常識だ。- It’s common knowledge that bears are carnivores. 
• 悟空が強いということはわすれない – I’ll never forget that Goku is strong.

See also: ことができる