の: connecting nouns
の does more than express possession. The second noun is the main item; the first adds a relationship such as owner, field, or location.
Find the main noun
私のかばん means 'my bag': かばん is the main noun and 私 identifies its owner. 日本語の本 means a Japanese-language book or a book about Japanese, with の expressing a language or subject relationship.
The meaning depends on the nouns and context. 学校の先生 refers to a teacher associated with a school; の itself does not describe the action of owning someone or something.
Follow longer noun phrases
友達の日本語の本 can mean 'my friend's Japanese-language book.' First group 日本語の本, then connect 友達の to identify whose book it is. This makes the structure easier to follow.
Long chains of nouns can leave relationships unclear. When writing, check whether the reader can tell which noun each phrase describes. If necessary, split the information into two shorter sentences.
の standing in for a noun
In これは私のです。, the item is already clear, so the noun after 私の is omitted. It means 'This is mine.' Understand the missing item from the preceding context.
Do not insert の before every modified noun. 静かな部屋 means 'a quiet room,' and 昨日買った本 means 'the book I bought yesterday.' Adjective and verb modifiers follow patterns different from noun + の.
Check: In 日本語の先生, is the main noun 日本語 or 先生?
Show the answer and why
The main noun is 先生. 日本語の specifies the subject taught, giving the meaning 'Japanese-language teacher.'