こと

こと has two main functions: (1) as a noun meaning 'thing, matter, affair' for abstract concepts, and (2) as a grammatical nominalizer that turns verbs/adjectives into noun phrases.

こと VS もの:

• こと: abstract things (events, facts, situations)
• もの: concrete, tangible things

Key grammar patterns

• ことがある: to have the experience of (past tense + ことがある)
• ことができる: to be able to
• ことにする: to decide to
• ことになる: it has been decided that
• ということ: the fact that, I heard that

NOMINALIZER こと VS の:
Both turn a clause into a noun phrase, but they are not interchangeable. こと is required before できる, にする, and になる, and it suits general or abstract statements. の is required with verbs of direct perception — 見(み)る, 聞(き)く — and is preferred for a concrete event the speaker witnessed: 彼(かれ)が走(はし)るのを見(み)た (I saw him run).

Writing

Sometimes written 事(こと), but hiragana is more common, especially in grammatical uses.