の
's, of (linking nouns)
Noun + の + Noun
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の joins two nouns so that the first describes the second. Often it's possession, like 's: 私の本, "my book"; 田中さんの車, "Mr Tanaka's car".
It covers much more than ownership, though: 日本語の本 is a Japanese book (or a book about Japanese), 大学の先生 is a university teacher, 三時の電車 is the three o'clock train.
The order is always describer first, thing described last. That's the reverse of English "of": 東京の駅 is "the station in Tokyo".
When the second noun is obvious you can leave it out: これは私のです, "this is mine".
Examples
これは私の本です。
This is my book.
田中さんは日本語の先生です。
Mr Tanaka is a Japanese teacher.
母の車は赤いです。
My mother's car is red.
七時の電車に乗ります。
I'll take the seven o'clock train.
そのかさは誰のですか。
Whose umbrella is that?