In short

English attaches a clause to a noun with one small word - “the man who works here”, “the book that I read” - and English is even happy to drop it (“the book I read”). Arabic makes two decisions instead of one. First, is the noun you are describing definite or indefinite? A definite noun takes an اسْم مَوْصول, a relative pronoun chosen for its gender and number: الرَّجُلُ الَّذي يَعْمَلُ هُنا. An indefinite noun takes nothing at all - the clause is simply hung on the noun the way an adjective is: رَأَيْتُ رَجُلًا يَعْمَلُ هُنا. Writing الَّذي after an indefinite noun is the single error this lesson exists to prevent.

The second half is the pronoun that English throws away and Arabic never does: العائِد, “the returner”. The clause after the مَوْصول has to be complete, with a pronoun inside it pointing back at the noun. “The book I read” is الكِتابُ الَّذي قَرَأْتُهُ - literally “the book which I read it”. And because an Arabic preposition can never be left dangling at the end the way English leaves it in “the house I live in”, the preposition goes into the clause too, with a pronoun stuck to it: البَيْتُ الَّذي أَسْكُنُ فيهِ.

The rule

1. The forms, and the one that moves. There are six shapes, matching the head noun in gender and number:

• singular: الَّذي (m.) / الَّتي (f.)
• dual: اللَّذانِ ~ اللَّذَيْنِ (m.) / اللَّتانِ ~ اللَّتَيْنِ (f.)
• plural: الَّذينَ (m.) / اللّاتي، اللَّواتي (f.)

All of them except the dual are مَبْنِيّ - frozen. الَّذي is الَّذي whether the phrase is the subject, the object or after a preposition; you never put a case vowel on it. Only the dual declines, with the ordinary dual endings of lesson 12: اللَّذانِ when the head is مَرْفوع, اللَّذَيْنِ when it is مَنْصوب or مَجْرور (جاءَ الطّالِبانِ اللَّذانِ نَجَحا, but سَأَلْتُ الطّالِبَيْنِ اللَّذَيْنِ نَجَحا). Why the dual behaves this way, and what “مَبْنِيّ” means in full, is lesson 40.

2. Watch the spelling of the ل. The written forms are not as regular as they sound. الَّذي، الَّتي and الَّذينَ are written with one ل carrying a shadda; the duals اللَّذانِ، اللَّتانِ and the feminine plurals اللّاتي، اللَّواتي are written with two. This is a fixed orthographic quirk, not a rule you can derive, and *اللَّذي or *الَّذانِ is a spelling error even though nothing about the pronunciation changes. Copy the table in the next section by hand once and it will stick.

3. The rule that carries the lesson: definite head, مَوْصول; indefinite head, nothing. This is the same agreement reflex you already have from lesson 8. An adjective copies its noun’s definiteness (بَيْتٌ كَبيرٌ / البَيْتُ الكَبيرُ), and a relative clause is a describer too. The مَوْصول is a definite word - it begins with الـ - so the noun it attaches to has to be definite too:

• definite head: رَأَيْتُ الرَّجُلَ الَّذي يَعْمَلُ هُنا. - “I saw the man who works here.”
• indefinite head: رَأَيْتُ رَجُلًا يَعْمَلُ هُنا. - “I saw a man who works here.”

In the second sentence there is no relative pronoun anywhere, and none is missing: the clause يَعْمَلُ هُنا is a صِفَة, a describing clause, and it does the same job that كَبيرًا would do in رَأَيْتُ رَجُلًا كَبيرًا. English learners find this hard because English needs “who” in both sentences. Arabic needs it in neither - it needs a match.

The headline mistake is therefore *رَأَيْتُ رَجُلًا الَّذي يَعْمَلُ هُنا: an indefinite noun followed by a definite describer, exactly as broken as *بَيْتٌ الكَبيرُ.

The traditional formula is worth memorizing as it stands: الجُمَلُ بَعْدَ النَّكِراتِ صِفاتٌ، وبَعْدَ المَعارِفِ أَحْوالٌ - “clauses after indefinites are descriptions, after definites they are circumstantials”. The second half explains what happens if you drop the مَوْصول after a definite noun: رَأَيْتُ الرَّجُلَ يَعْمَلُ هُنا is not wrong, but it no longer means “the man who works here”. It means “I saw the man while he was working here” - a حال, lesson 55. The مَوْصول is what keeps the clause attached to the noun instead of to the event.

4. “Definite” includes more than الـ. Anything definite counts, so the مَوْصول is required after a noun made definite by a possessive suffix (lesson 7), by an إِضافَة (lesson 20), or by being a name:

• كِتابي الَّذي قَرَأْتُهُ فِي الصَّيْفِ - “my book that I read in the summer”
• سَيّارَةُ أَخي الَّتي اِشْتَراها أَمْسِ - “my brother’s car, which he bought yesterday”
• القاهِرَةُ الَّتي زُرْتُها - “the Cairo I visited”

5. Agreement, and the non-human plural. The مَوْصول copies the head’s gender and number, with one exception that is not an exception at all once you remember lesson 11: a plural of non-human things counts as feminine singular, so it takes الَّتي, never الَّذينَ:

• الكُتُبُ الَّتي اِشْتَرَيْتُها - “the books I bought”
• المُدُنُ الَّتي زُرْناها - “the cities we visited”
• but المُوَظَّفونَ الَّذينَ يَعْمَلونَ هُنا - “the employees who work here”

الَّذينَ is for human males and for mixed groups; اللّاتي and اللَّواتي are for groups of women. Both feminine plurals are current in MSA and interchangeable; pick اللّاتي and stay with it.

6. العائِد - the clause must point back. After the مَوْصول comes جُمْلَةُ الصِّلَة, and it must be a complete clause containing a pronoun that refers to the head. Which pronoun depends on the job the head does inside that clause:

• the head is the subject - the pronoun is already inside the verb, so nothing is written: الرَّجُلُ الَّذي يَعْمَلُ هُنا. The ي of يَعْمَلُ is the reference.
• the head is the direct object - attach the object pronoun of lesson 29 to the verb: الكِتابُ الَّذي قَرَأْتُهُ، المَقالَةُ الَّتي كَتَبْتُها.
• the head follows a preposition - preposition plus pronoun: البَيْتُ الَّذي أَسْكُنُ فيهِ، الرَّجُلُ الَّذي تَكَلَّمْتُ مَعَهُ، المُشْكِلَةُ الَّتي تَحَدَّثْنا عَنْها.
• the head is the possessor - the pronoun sits on the noun it owns: الطّالِبَةُ الَّتي أَبوها طَبيبٌ، الرَّجُلُ الَّذي بَيْتُهُ كَبيرٌ. This is how Arabic says “whose”.

Only the first case looks empty to an English eye, and it is not: an Arabic verb carries its subject. In every other case, modern written Arabic keeps the pronoun; leave it out and the sentence reads as unfinished. *الكِتابُ الَّذي قَرَأْتُ stops mid-thought: “the book which I read…”.

7. A preposition can never strand. English happily ends a relative clause with a bare preposition - “the house I live in”, “the man I spoke to”, “the problem we talked about”. Arabic has no such position. The preposition stays with its object, and its object here is the عائِد pronoun, so the two are written as one word:

*الرَّجُلُ الَّذي تَكَلَّمْتُ مَعَ is the classic transfer error from English and is not a sentence at all - مَعَ without a following word is half a word.

8. Indefinite heads need the عائِد too. Dropping the مَوْصول does not mean dropping the pronoun. The clause still has to be complete:

• قَرَأْتُ مَقالًا كَتَبَهُ صَحَفِيٌّ مِصْرِيٌّ. - “I read an article written by an Egyptian journalist.”
• أَسْكُنُ في بَيْتٍ نافِذَتُهُ كَبيرَةٌ. - “I live in a house whose window is big.”

So the two rules are independent: the مَوْصول tracks definiteness, the عائِد tracks the role the head plays inside its own clause.

9. What the صِلَة can be. Usually a verbal sentence, but a nominal sentence and a bare prepositional phrase work just as well:

• verbal: الطّالِبُ الَّذي نَجَحَ فِي الاِمْتِحانِ
• nominal: الطّالِبُ الَّذي أَبوهُ مُهَنْدِسٌ
• prepositional: الطّالِبُ الَّذي فِي الغُرْفَةِ

What it can never be is a fragment with a hole in it. If you can still ask “which one?” after reading the clause, something is missing.

10. Left for later. The مَوْصول can also stand with no head noun in front of it - “the one who”, “what” - using مَنْ and ما (مَنْ يَدْرُسُ يَنْجَحُ). Those free relatives, and the fused forms مِمَّنْ، عَمّا، مِمّا, are lesson 68. The dual’s endings and the meaning of مَبْنِيّ belong to lesson 40. And the حال reading of a clause after a definite noun - the second half of the formula in rule 3 - is lesson 55.

Forms & patterns

الأَسْماءُ المَوْصولَة - the whole set. Read the two dual rows as one item that changes shape; everything else is one fixed word:

Left out here: the further feminine plurals اللّائي - still current in formal writing - and the much rarer اللّاءِ, which you will meet in reading but need not write yourself, and the free-standing مَنْ and ما of lesson 68.

Written with one ل or with two. No rule, just the list:

The decision that starts every relative clause:

العائِد, by the job the head does inside the clause:

Prepositions with their عائِد pronouns - learn these as ready-made tails:

The same clause on a definite and an indefinite head - drill these in pairs, out loud:

Examples

Common mistakes

• رَأَيْتُ رَجُلًا الَّذي يَعْمَلُ هُنا. -> رَأَيْتُ رَجُلًا يَعْمَلُ هُنا. - the headline error. An indefinite head takes no مَوْصول at all; the clause simply follows, like an adjective.
• الرَّجُلُ يَعْمَلُ هُنا صَديقي. (meaning “the man who works here is my friend”) -> الرَّجُلُ الَّذي يَعْمَلُ هُنا صَديقي. - the mirror error. Without الَّذي a definite head plus a clause is read as a حال, “the man, while working here…”, and the sentence falls apart.
• الكِتابُ الَّذي قَرَأْتُ مُفيدٌ. -> الكِتابُ الَّذي قَرَأْتُهُ مُفيدٌ. - the عائِد is obligatory. English drops “it”, modern Arabic keeps it: “the book which I read it”.
• الرَّجُلُ الَّذي تَكَلَّمْتُ مَعَ. -> الرَّجُلُ الَّذي تَكَلَّمْتُ مَعَهُ. - an Arabic preposition cannot be stranded. Whatever English leaves at the end, Arabic writes with a pronoun glued to it.
• البَيْتُ الَّذي أَسْكُنُ كَبيرٌ. -> البَيْتُ الَّذي أَسْكُنُ فيهِ كَبيرٌ. - here the whole preposition went missing. سَكَنَ normally reaches its place through في, and the عائِد rides on it.
• الكُتُبُ الَّذينَ اِشْتَرَيْتُهُمْ. -> الكُتُبُ الَّتي اِشْتَرَيْتُها. - a non-human plural is feminine singular for every kind of agreement (lesson 11), so both the مَوْصول and the عائِد go to the ـها form.
• الطّالِباتُ الَّذينَ نَجَحْنَ. -> الطّالِباتُ اللّاتي نَجَحْنَ. - الَّذينَ is masculine. A group of women takes اللّاتي (or اللَّواتي).
• الطّالِبانِ الَّذينَ نَجَحا. -> الطّالِبانِ اللَّذانِ نَجَحا. - the dual has its own form, and it is not the plural one.
• سَأَلْتُ الطّالِبَيْنِ اللَّذانِ نَجَحا. -> سَأَلْتُ الطّالِبَيْنِ اللَّذَيْنِ نَجَحا. - the dual is the only مَوْصول that declines: اللَّذانِ goes with a مَرْفوع head, اللَّذَيْنِ with a مَنْصوب or مَجْرور one.
• اللَّذي قَرَأْتُهُ -> الَّذي قَرَأْتُهُ - a spelling slip worth naming: the singular and الَّذينَ are written with one ل, the duals and اللّاتي with two.
• المَرْأَةُ الَّذي تَعيشُ فِي القاهِرَةِ. -> المَرْأَةُ الَّتي تَعيشُ فِي القاهِرَةِ. - the مَوْصول copies the head’s gender, exactly as an adjective does.

Tips

• Before you write anything, ask one question: does the head carry الـ, a possessive suffix, or a name? Yes → مَوْصول. No → nothing. That single check removes the commonest error.
• Reuse the reflex from lesson 8. الَّذي is a definite word, and definiteness has to match; a clause after a noun is just a longer adjective.
• Then ask the second question: what job does the head do inside its own clause? If it is not the subject, you owe the clause a pronoun.
• Translate English “the man I spoke to” by moving the stranded preposition inside and giving it a pronoun - مَعَهُ، عَنْهُ، فيهِ، عَلَيْها. Collect these tails as whole words.
• For anything non-human and plural, one switch flips everything: الَّتي plus ـها. Books, cities, cars, ideas - all of them.
• Say each new sentence back to yourself in literal English (“the book which I read it”). If the literal version has no extra pronoun in it, the Arabic is probably incomplete.
• Copy the six forms out by hand once, with their shaddas and their one-or-two ل. The shape is the only part that has to be memorized; the usage is a two-question rule.
• Ignore مَنْ and ما for now. Every relative clause you build at this level hangs on a noun that is actually written.