In short

English has two ways to link a thing to its owner: “the house’s door” and “the door of the house”. Arabic has one, and it uses no linking word at all - you simply set the two nouns side by side in the order possessed-then-possessor: بابُ البَيْتِ. There is no word for “of” and no apostrophe; the whole grammar of the phrase is carried by the endings. This construction is called الإِضافَة, and it is everywhere: addresses, job titles, room names, dates, headlines. You cannot say “the university library” or “my brother’s car” without it.

What you have to learn is not the word order - that is one sentence long - but the three things the construction does to the endings. The first noun loses its article and its tanwīn, however definite it is. The second noun is locked into the genitive and never leaves it. And the whole phrase counts as definite or indefinite according to the second word, not the first. Get those three right and the rest of Arabic possession is free.

The rule

1. The two positions have names. The first noun - the thing possessed - is the مُضاف. The second noun - the possessor - is the مُضافٌ إِلَيْهِ. Word order is fixed and there is nothing between them:

• بابُ البَيْتِ - the door of the house (مُضاف = بابُ، مُضافٌ إِلَيْهِ = البَيْتِ)
• سَيّارَةُ المُديرِ - the manager’s car
• كِتابُ الطّالِبِ - the student’s book

English reads the two words in reverse (“the student’s book”), or in the same order with “of” inserted (“the book of the student”). Arabic inserts nothing. Note also that the relationship does not have to be ownership: غُرْفَةُ الطَّعامِ is a room for food, خاتَمُ ذَهَبٍ is a ring made of gold, بابُ البَيْتِ is a part of a whole. Arabic uses one construction for the lot.

2. Iron rule one: the مُضاف takes no الـ and no tanwīn. This is the rule learners break most often, because English says “the door of the house” and the hand reaches for the article. The first word of an إِضافَة is bare: no الـ on the front, no doubled sign at the end, just a single case vowel.

• بابٌ (a door) + البَيْتِ -> بابُ البَيْتِ - never *بابٌ البَيْتِ
• البابُ (the door) + البَيْتِ -> بابُ البَيْتِ - never *البابُ البَيْتِ

That single bare vowel is a signal you can read: a noun carrying one vowel, no article and no tanwīn, is nearly always announcing that another noun is coming right behind it. (The one common exception is the diptote, which refuses tanwīn for reasons of its own - lesson 41.)

3. Iron rule two: the مُضافٌ إِلَيْهِ is always مَجْرور. Whatever is happening in the rest of the sentence, the second term sits in the genitive and stays there. Its ending is a kasra - ـِ if it is definite, ـٍ if it is indefinite:

• بابُ البَيْتِ / بابُ بَيْتٍ
• سَيّارَةُ المُديرِ / سَيّارَةُ مُديرٍ

This is the one fixed point in the phrase. No verb, no preposition and no case in the outer sentence can move it. (One class of nouns shows this genitive with a fatḥa instead of a kasra - دِمَشْقَ below. Those are the diptotes of lesson 41; for now just accept the form as given.)

4. Iron rule three: definiteness comes from the second term. The مُضاف cannot carry الـ, so it cannot announce its own definiteness. The second word decides for the whole phrase:

• بابُ البَيْتِ - the door of the house (definite: the second term has الـ)
• بابُ بَيْتٍ - a door of a house (indefinite: the second term has tanwīn)

A proper name or a pronoun in second position also makes the phrase definite, because names and pronouns are definite in themselves: سَيّارَةُ مُحَمَّدٍ (Muhammad’s car), كِتابُهُ (his book). That last one is worth pausing on - every attached pronoun you learned in lesson 7 is the second term of an إِضافَة. You have been building constructs since then without being told.

5. Only the first word moves through the cases. Because the second term is frozen in the genitive, the outer sentence acts on the مُضاف alone. Take one construct and put it in all three slots:

• هٰذا بابُ البَيْتِ. - the خَبَر of a nominal sentence, so مَرْفوع
• أَغْلَقْتُ بابَ البَيْتِ. - direct object, so مَنْصوب
• المِفْتاحُ في بابِ البَيْتِ. - after a preposition, so مَجْرور

Three sentences, one word changing. Drill this trio out loud; it is the whole case behaviour of the construction.

6. Chains: any number of links, one article at the end. The مُضافٌ إِلَيْهِ can itself be the مُضاف of a further pair. Every middle word then obeys both rules at once - it is genitive (because it follows a مُضاف) and it is bare of article and tanwīn (because another noun follows it):

مِفْتاحُ بابِ بَيْتِ المُديرِ - the key to the door of the manager’s house

Only the last word may carry الـ, and only the last word decides the definiteness of the whole chain. Read a chain from its first word, adding “of” at each step. Chains of three or four are normal in writing; longer ones, and what happens when a quantifier joins in, are lesson 42.

7. The ة is heard as a ت inside an إِضافَة. A feminine first term keeps its ة in writing, but because the case vowel after it is now obligatory, the letter is pronounced as a full ت: مَدينَةُ دِمَشْقَ، مَكْتَبَةُ الجامِعَةِ، غُرْفَةُ الطَّعامِ. Said on its own, at the end of an utterance, مَدينَة is pronounced with a light final -a and the ت is not heard at all (that is the pausal form, which the pronunciation guide sets out in full). Two consequences:

• Do not rewrite the letter. It stays ة: مَدينَةُ دِمَشْقَ, never *مَدينَتُ دِمَشْقَ. Before a following noun the ة is only heard as ت. It is actually written ت in the dual (مَدينَتانِ) and before an attached pronoun (مَدينَتُها), but not here.
• Do not swallow it in speech. مَكْتَبَة الجامِعَة with two silent endings is dialect, not الفُصْحى.

8. The dual and the sound masculine plural drop their ن. Both endings end in a ن that exists only when the word stands free. As soon as the word becomes a مُضاف, the ن falls off:

• مُعَلِّمانِ -> مُعَلِّما المَدْرَسَةِ - the school’s two teachers
• مُعَلِّمَيْنِ -> مُعَلِّمَيِ المَدْرَسَةِ - the same, in the accusative or genitive
• مُعَلِّمونَ -> مُعَلِّمو المَدْرَسَةِ - the school’s teachers
• مُعَلِّمينَ -> مُعَلِّمي المَدْرَسَةِ - the same, in the accusative or genitive

The ـَيْ of the dual picks up a kasra when it runs straight into الـ, which is why you see مُعَلِّمَيِ. Two endings do not drop anything: the sound feminine plural simply loses its tanwīn like any other noun (مُعَلِّماتٌ -> مُعَلِّماتُ المَدْرَسَةِ), and so does a broken plural (كُتُبٌ -> كُتُبُ الطُّلّابِ).

9. An adjective goes after the whole construct - and its case tells you what it describes. Nothing may be wedged between the two terms, so a describing word has to wait until the construct is finished. That leaves a genuine ambiguity in English, which Arabic usually resolves with the ending:

• بابُ البَيْتِ الكَبيرُ - the big door of the house (nominative, matching بابُ)
• بابُ البَيْتِ الكَبيرِ - the door of the big house (genitive, matching البَيْتِ)

The adjective still obeys every rule of lesson 8: it agrees in gender, number, case and definiteness. Since a definite construct makes its first term definite too, an attributive adjective takes الـ either way - so the case vowel is often the only signal left, which is one more reason this path prints the endings. Gender and number help when they can: سَيّارَةُ المُديرِ الجَديدَةُ is the manager’s new car, سَيّارَةُ المُديرِ الجَديدِ is the new manager’s car. When the two terms happen to share their case as well - after a preposition, say - the phrase is simply ambiguous, and only the context decides.

If the construct is indefinite, the adjective is indefinite as well: بابُ بَيْتٍ كَبيرٌ (a big door of a house) against بابُ بَيْتٍ كَبيرٍ (a door of a big house).

10. What an إِضافَة is not. Two nouns in a row are not always a construct. If the first one carries الـ or tanwīn, you are looking at something else - a subject and its predicate (البَيْتُ كَبيرٌ), or a noun and its adjective (البَيْتُ الكَبيرُ). And Arabic does not translate English “of” with مِنْ in this sense: *بابٌ مِنَ البَيْتِ is not “the door of the house”.

Forms & patterns

One construct, all three slots - only the first word moves:

Definiteness is decided by the second term:

What entering the construct does to the first term:

Everyday pairs worth learning as single vocabulary items:

Chains - every middle word is bare and genitive, only the last word carries الـ:

The adjective after the construct, and what its ending points at:

Examples

Common mistakes

• البابُ البَيْتِ -> بابُ البَيْتِ - the first term of an إِضافَة never takes الـ, even when English says “the”. Its definiteness comes from the word after it.
• بابٌ البَيْتِ -> بابُ البَيْتِ - and it never takes tanwīn either. The مُضاف ends in one bare vowel, always.
• بابُ البَيْتُ -> بابُ البَيْتِ - the second term is مَجْرور no matter what the sentence around it is doing. It is the fixed point of the construction.
• بابُ الكَبيرُ البَيْتِ -> بابُ البَيْتِ الكَبيرُ - nothing may stand between the two terms. The adjective waits until the construct is complete.
• بابٌ مِنَ البَيْتِ -> بابُ البَيْتِ - Arabic does not translate the English “of” with a preposition here. The link is the bare word order and the genitive.
• مَدينَةٌ دِمَشْقَ -> مَدينَةُ دِمَشْقَ - a feminine first term drops its tanwīn like any other, and the ة is then pronounced as a full ت.
• مَدينَتُ دِمَشْقَ -> مَدينَةُ دِمَشْقَ - but do not rewrite the letter. The ة is heard as a ت and still written as a ة; it is really written ت only in the dual (مَدينَتانِ) and before an attached pronoun (مَدينَتُها).
• مُعَلِّمونَ المَدْرَسَةِ -> مُعَلِّمو المَدْرَسَةِ - the ن of the sound masculine plural drops before the second term.
• مُعَلِّمانِ المَدْرَسَةِ -> مُعَلِّما المَدْرَسَةِ - the same for the dual, in both its forms: مُعَلِّما and مُعَلِّمَيِ.
• بابُ البَيْتِ كَبيرٌ (meaning “the big door of the house”) -> بابُ البَيْتِ الكَبيرُ - a definite construct takes a definite adjective. Without الـ the words are a full sentence: “the door of the house is big”.

Tips

• Learn to read the signal, not just the rule: a noun with a single case vowel, no article and no tanwīn is nearly always telling you that another noun is coming. Do not stop at it.
• Translate a construct backwards into English and check that it makes sense: بابُ البَيْتِ -> “door - of the house”. If your English needs “of”, your Arabic needs nothing.
• Drill the three-slot trio (هٰذا بابُ البَيْتِ / أَغْلَقْتُ بابَ البَيْتِ / في بابِ البَيْتِ) with five different constructs. When only the first word moves without thinking, you have it.
• Store real compounds as whole vocabulary items - غُرْفَةُ النَّوْمِ، رَقْمُ الهاتِفِ، نِهايَةُ الأُسْبوعِ - rather than assembling them each time. Half of everyday Arabic is إِضافات.
• You already know one kind: كِتابي، بَيْتُكَ، سَيّارَتُها. An attached pronoun is a second term, which is exactly why those words refuse الـ.
• When you want to attach two things to one owner, do not try to coordinate two first terms. Finish the construct, then continue with a pronoun: بابُ البَيْتِ ونافِذَتُهُ. The tidier ways of packing more into one phrase are lesson 42.
• In unvocalized text the endings vanish, but the construct is still visible: a missing ن on a plural (معلمو المدرسة), or a bare first noun followed by one carrying ال (باب البيت), give it away.
• Do not worry yet about second terms that take a fatḥa instead of a kasra - دِمَشْقَ، مِصْرَ، فاطِمَةَ. They are the diptotes of lesson 41; copy the form for now.