In short

To say “the book that I read”, Japanese puts a whole clause in the plain form directly in front of the noun: 私が読んだ本 (watashi ga yonda hon). There is no word for “who”, “which” or “that”, nothing marks the head noun, and nothing agrees with anything. One rule - plain clause + noun - covers every English relative clause you will ever need.

The rule

English builds a relative clause with machinery: a relative pronoun (who / which / that), a fixed position after the noun, and sometimes a stranded preposition (“the town I live in”). Japanese throws all of that away.

The single rule: a plain-form clause slots in front of the noun, exactly where an adjective would go.

[ Clause in plain form ] + Noun

Compare, in order:

• 高い + 本 -> 高い本 (takai hon) - an expensive book
• 私が読んだ + 本 -> 私が読んだ本 (watashi ga yonda hon) - a book I read

Grammatically these are the same shape. A modifying clause is just a bigger adjective.

Step 1 - Put the clause in the plain form. This is the whole reason lesson 17 came first. Even when the sentence ends politely in です／ます, the verb inside the modifier is plain: 読む, 読まない, 読んだ, 読まなかった. Writing ×読みました本 is one of the most visible learner errors in Japanese.

Step 2 - Mark the subject inside the clause with が, never は. は is a sentence-level topic marker, and a modifier has no topic of its own to mark. 私が読んだ本 is correct, ×私は読んだ本 is not. This is the cleanest, most reliable は／が rule in the language - inside a noun-modifying clause the subject is が (or の), never a topic は. (は does survive in some other, more loosely attached kinds of subordinate clause, such as から and けど clauses - but contrast is the only thing that ever pulls a は into a modifier, so at this stage treat the modifier as が-only. Why is deferred to は and が revisited.)

Step 3 - Leave a gap. Do not repeat the head noun. The head noun plays a role inside the clause, but that role is simply left empty - no pronoun, no particle. English “the town which I live in” becomes 私が住んでいる町, with nothing at all standing in for 町. Say ×私が町に住んでいる町 and you have said the noun twice.

The head noun’s own particle belongs to the outer sentence. Whatever case the head noun needs, it takes after the modifier, as if the modifier were not there: 私が読んだ本を買いました (I bought the book I read), 私が住んでいる町は静かです (the town I live in is quiet). The modifier never changes shape to suit the head.

が -> の inside the clause. In a short modifying clause the subject が may be replaced by の with no change in meaning:

• 私が読んだ本 = 私の読んだ本 (watashi no yonda hon)
• 父が撮った写真 = 父の撮った写真 (chichi no totta shashin)

The の version is compact and slightly bookish. It only works inside a modifier: in a main clause の can never mark the subject, so 私が本を読みました simply has no の version (私の本を読みました is a different sentence - I read my book, with 私の modifying 本). It also gets less natural as the clause grows: 私が昨日図書館で借りた本 is neutral, while ?私の昨日図書館で借りた本 already feels strained. Rule of thumb: short clause, use either; long clause, use が.

Not only verbs. い-adjectives, な-adjectives and nouns modify a noun in their attributive form, and this is where な-adjectives and nouns differ from the plain form you learned in lesson 17. A な-adjective ends its non-past in な, not だ; a noun links with の, not だ:

• 静かな所 (shizuka na tokoro) - a quiet place
• 日本語が上手な人 (Nihongo ga jōzu na hito) - a person who is good at Japanese
• 病気の人 (byōki no hito) - a person who is ill

Tense inside the clause is relative, not absolute. The plain past た inside a modifier means “before the event of the main clause”, and the non-past means “at or after it” - not “past/present relative to now”. 明日会う人 (ashita au hito) is the person I will meet tomorrow even though 会う is non-past; 去年住んでいた町 is the town I was living in last year. Choose the form by comparing it to the main verb, not to the moment of speaking.

Time and reason heads work the same way. 日本に来たとき (Nihon ni kita toki) - when I came to Japan - is nothing more than a modifying clause in front of the noun とき. The same for 理由 (riyū, “reason”), 場合 (baai, “case”). Because the tense choice before とき carries extra meaning, it gets its own treatment in Temporal clauses.

Stacking and embedding. One modifier can be long: 父が去年京都で撮った写真 packs a subject, a time and a place into a single clause. Two modifiers can also stack on one noun: 母が作ったおいしい料理 (the tasty food my mother made). A modifier inside another modifier is also legal - 私が読んだ本を書いた人 (the person who wrote the book I read) - but each level forces the listener to hold an unfinished clause in memory, and speech rarely goes past one embedding. When a sentence gets that heavy, native writers split it in two.

Forms & patterns

What sits in front of the head noun, by word class. Note the な-adjective and noun rows - their non-past attributive form is not だ.

The head noun can play any role inside the clause. English needs a different pronoun (or a stranded preposition) for each row; Japanese does not change at all.

Word order against English. Read a Japanese noun phrase from the head noun backwards.

が and の inside the clause.

Examples

Common mistakes

• 昨日買いましたかばんです。 (Kinō kaimashita kaban desu.) -> 昨日買ったかばんです。 (Kinō katta kaban desu.) - the verb inside a modifier is always plain, even though the sentence itself ends politely. Politeness lives on the final predicate only.
• これは私は昨日読んだ本です。 (Kore wa watashi wa kinō yonda hon desu.) -> これは私が昨日読んだ本です。 (Kore wa watashi ga kinō yonda hon desu.) - は marks the topic of the whole sentence, and a modifying clause has no topic of its own. Subordinate subject = が.
• 私が町に住んでいる町は静かです。 (Watashi ga machi ni sunde iru machi wa shizuka desu.) -> 私が住んでいる町は静かです。 (Watashi ga sunde iru machi wa shizuka desu.) - do not repeat the head noun inside the clause. Japanese leaves the slot empty; there is no resumptive pronoun.
• 日本語を教えているの先生 (Nihongo o oshiete iru no sensei) -> 日本語を教えている先生 (Nihongo o oshiete iru sensei) - a verb clause attaches to the noun bare. The の of 私の本 joins two nouns and has no business after a verb.
• 病気な人 (byōki na hito) -> 病気の人 (byōki no hito) - 病気 is a plain noun, so it links with の. Only true な-adjectives take な: 静かな所, 上手な人.

Tips

• Build and read these backwards. Take the head noun first (本), then pile the description in front of it: 読んだ本 -> 私が読んだ本 -> 私が昨日読んだ本. English grows to the right, Japanese grows to the left, and the noun you are actually talking about is always the last word of the phrase.
• Ask “what particle is missing?” Nearly every modifier has one empty slot. In 私が読んだ本 the gap is を (私が本を読んだ); in 私が住んでいる町 it is に (私が町に住んでいる). Locating that gap tells you instantly what the phrase means - and stops you from filling it in. (Heads like とき, 理由 and 場合 are the exception: they name the situation the clause describes instead of filling a slot inside it.)
• た inside a modifier is not “past”. It means “already finished, relative to the main verb”. 買った本 in a future sentence is the book I will have bought. Judge the form against the main clause, never against now.
• One embedding is the practical ceiling. 私が読んだ本を書いた人 parses, but a second layer will lose your listener. Native speakers break long descriptions into two sentences - so should you.