In short

English puts a small word in front of the noun: “a house”, “the house”. Arabic marks the same difference in two other places - at the very end of the noun and at its very front - with two signs that can never sit on the same word. The doubled vowel sign called tanwīn says the noun is indefinite: بَيْتٌ. The prefix الـ says it is definite: البَيْتُ. You choose one. Writing both, as in *الْبَيْتٌ, is one of the commonest beginner errors, and it is not a small slip: it is like writing “a the house”.

الـ is spelled identically in front of every noun, but it is not always heard the same way. In front of fourteen letters called حُروف شَمْسِيَّة the ل goes silent and the following consonant doubles instead - الشَّمْسُ. In front of the other fourteen, the حُروف قَمَرِيَّة, the ل is pronounced normally - القَمَرُ. On top of that, a whole family of nouns is already definite without الـ: names, pronouns, demonstratives, and anything carrying a possessive ending.

The rule

1. One noun, one mark. Definiteness is a two-way switch, and Arabic will not let you throw both levers at once:

• indefinite: بَيْتٌ (a house), كِتابٌ (a book), مَدْرَسَةٌ (a school)
• definite: البَيْتُ (the house), الكِتابُ (the book), المَدْرَسَةُ (the school)

Notice what happens to the ending. The indefinite noun ends in the doubled sign ـٌ; the definite noun ends in the single sign ـُ. Tanwīn is exactly the case vowel plus an ن sound that is heard but never written as a letter. So *الْبَيْتٌ and *الْكِتابًا are impossible: the moment الـ goes on the front, the ending loses its second stroke.

2. Tanwīn exists in all three cases. The lesson before this one introduced the three noun endings. Each of them has an indefinite twin, and the pairing is completely regular:

Do not think of tanwīn as “the ending for indefinite nouns” in the singular; think of it as the whole set ـٌ ـً ـٍ, each one carrying its case and the message “indefinite”. Which case a noun takes in a given sentence is decided by the sentence, not by this lesson.

3. الـ never changes its spelling. Whatever follows it - a sun letter, a moon letter, a hamza - you always write أَلِف + لام, joined to the noun with no space: البَيْتُ، الشَّمْسُ، الأُسْتاذُ. Arabic has no separate word for “the”, and no space is ever left after الـ.

4. Sun letters: the ل is written but not pronounced. Fourteen consonants swallow the ل of the article. When one of them follows الـ, the ل is not heard at all; instead the sun letter itself is doubled, and you mark that doubling with a shadda:

الشَّمْسُ، الرَّجُلُ، النّورُ، السَّلامُ، الطّالِبُ، الدَّرْسُ، الزَّيْتُ، الصَّديقُ

In fully vocalized writing the ل of a sun-letter article carries no sign at all - no vowel and no sukūn - because there is nothing there to pronounce. The shadda on the next letter is your instruction: hold that consonant twice as long.

5. Moon letters: the ل is pronounced. The other fourteen leave the article intact. Here the ل is heard and the following letter is single. Strict vocalization puts a sukūn on that ل - الْقَمَرُ - though most printing, this lesson’s lists included, leaves it unmarked:

القَمَرُ، الكِتابُ، البَيْتُ، المَدْرَسَةُ، الوَلَدُ، الجَبَلُ، العَيْنُ، الهاتِفُ

The two names come from the two example words themselves: شَمْس begins with a sun letter and قَمَر with a moon letter, so each word demonstrates its own class.

6. Why the split is not arbitrary. Every one of the fourteen sun letters is produced with the tip or the blade of the tongue against the teeth or the ridge behind them - the very same region where ل is made. A consonant made in the same place simply absorbs its neighbour. The moon letters are made elsewhere: on the lips, in the throat, at the back of the mouth. So your tongue will usually predict the class for you - but the classes were fixed on an older pronunciation, so check the list where you are unsure. ج is the trap: today it is made at the front of the mouth like a sun letter, yet it is lunar - الجَبَلُ, never *الجَّبَل. And note the neatest case of all: ل is itself a sun letter, so it doubles into itself - اللُّغَةُ، اللَّيْلُ، اللَّوْنُ.

7. هَمْزَةُ الوَصْل: the alif drops after a preceding word. The alif of الـ is a “connecting” alif (the pronunciation guide’s hamza lesson). It is pronounced only when the article opens the utterance. As soon as another word comes in front of it, the alif is not pronounced at all and the article leans directly on the last vowel of that word - the two run together as one breath:

• قَرَأْتُ الْكِتابَ - the ending of قَرَأْتُ runs straight into the ل
• ذَهَبْتُ إِلَى الْمَدْرَسَةِ
• الوَلَدُ فِي الْبَيْتِ
• خَرَجْتُ مِنَ الْبَيْتِ - مِنْ ends in a consonant, so it borrows a vowel to link up. The vowel is a fatḥa, and مِنْ is the odd one out in that: almost every other sukūn-final word borrows a kasra - عَنِ الْمَكْتَبِ، هَلِ الْمِفْتاحُ جاهِزٌ؟، وَصَلَتِ الطّائِرَةُ - while the plural ـمْ of هُمْ and أَنْتُمْ borrows a ḍamma instead (lesson 6)

That sukūn on the ل is there in either position; what changes mid-sentence is only the alif, which stops being pronounced - so الْ inside a sentence is read straight off the previous word’s vowel. A word that ends in a long vowel borrows nothing: the long vowel is dropped instead, and vocalization shows what is left as a fatḥa on the ل of إِلَى and عَلَى - إِلَى الْمَدْرَسَةِ، عَلَى الصُّنْدوقِ - so the spelling never changes and only the mark is new. The Qur’an and other sacred texts mark the silent alif with a waṣla, the head of a small ص written over it (ٱ); ordinary printing, including this lesson, leaves the alif bare. With a sun letter nothing extra is visible at all, because the ل had no sign to begin with: رَأَيْتُ الشَّمْسَ.

8. Writing tanwīn fatḥ. The accusative tanwīn is the only one that changes the spelling of the word: an extra alif is added as a seat for it, and that alif is silent.

• كِتابًا، بَيْتًا، وَلَدًا، جَديدًا

Two positions are in use in print - over the last letter (كِتابًا) or over the added alif (كِتاباً). They are the same thing; write it the first way and read both. Three groups of words take no extra alif:

• words ending in ة: مَدْرَسَةً، سَيّارَةً، كَلِمَةً
• words ending in ء that already sits on a long alif: ماءً، سَماءً، دَواءً
• words ending in ى: فَتًى، مَعْنًى، مَقْهًى

But a hamza that does not follow a long alif behaves like any other consonant and does take the seat: شَيْئًا، جُزْءًا، عِبْئًا.

9. Definite without الـ. Do not reach for الـ every time English says “the”. Several classes of word are inherently definite and reject the article:

• proper names: مُحَمَّدٌ، فاطِمَةُ، مِصْرُ، لُبْنانُ - so *الْمُحَمَّد is wrong. (A number of place names do carry the article as part of the name itself: القاهِرَةُ، المَغْرِبُ، العِراقُ. Learn those as spelled.)
• pronouns: هُوَ، هِيَ، أَنا، أَنْتَ (lesson 6)
• demonstratives: هٰذا، هٰذِهِ، ذٰلِكَ (lesson 13)
• nouns with a possessive ending: كِتابي، بَيْتُكَ، سَيّارَتُها (lesson 7) - “my book” is already the specific one, so *الْكِتابي is impossible
• the first term of an إِضافَة: بابُ الْبَيْتِ، مِفْتاحُ السَّيّارَةِ (lesson 20) - the first word takes neither الـ nor tanwīn; it draws its definiteness from the second word, so بابُ بَيْتٍ is “a door of a house”

One honest wrinkle: a fully declinable proper name still shows tanwīn - مُحَمَّدٌ، خالِدٌ. There the tanwīn is only the ordinary full ending of the word; it is not saying “indefinite”, because a name points at one person by nature. This is the one place where tanwīn and definiteness coexist, and it costs you nothing at A1: just do not delete the tanwīn from names that have it.

10. Flag for later. A class of nouns called المَمْنوع مِنَ الصَّرْف never takes tanwīn at all, in any case: فاطِمَةُ، أَحْمَدُ، مَدارِسُ. Their endings work differently and lesson 41 handles them. For now, if a word you meet has no tanwīn where you expected one, suspect this group rather than a misprint.

Forms & patterns

The two marks side by side, with a feminine noun added so you can see that ة changes nothing about the system:

The fourteen حُروف شَمْسِيَّة - the ل disappears and the letter doubles:

The fourteen حُروف قَمَرِيَّة - the ل keeps its sukūn and is pronounced:

The article at the start of an utterance against the article inside one:

The accusative tanwīn and its seat:

Examples

Common mistakes

• الْبَيْتٌ -> البَيْتُ (the house) or بَيْتٌ (a house) - a noun can never carry the article and tanwīn at the same time. Decide which one you mean.
• هٰذا كِتابُ. -> هٰذا كِتابٌ. - an indefinite noun ends in the doubled sign, not a plain ḍamma. A single ـُ with no article means the word is definite by something else.
• الْشَّمْسُ -> الشَّمْسُ - before a sun letter the ل takes no sukūn, because it is not pronounced. Only the shadda on the ش is written.
• الشَمْسُ -> الشَّمْسُ - and do not leave the shadda out either; without it a reader has to guess, and a beginner will pronounce the ل.
• القَّمَرُ -> القَمَرُ - moon letters never take the article’s ل into themselves, so they never carry a shadda from it. The ل stays audible and the ق stays single.
• الْمُحَمَّدُ -> مُحَمَّدٌ - a personal name is definite in itself, so it refuses the article; and here the tanwīn is simply the word’s normal full ending.
• الْكِتابي -> كِتابي - a possessive ending already makes the noun definite; الـ and a suffix cannot share one word.
• الْبابُ الْبَيْتِ -> بابُ الْبَيْتِ - the first word of an إِضافَة takes neither الـ nor tanwīn, even when English says “the door of the house”.
• كِتابً -> كِتابًا - the accusative tanwīn needs its silent alif seat.
• مَدْرَسَةًا / ماءًا -> مَدْرَسَةً / ماءً - but never after ة, and never after a ء that already sits on a long alif.

Tips

• Store every new noun in your memory with its tanwīn attached: بَيْتٌ، كِتابٌ، مَدْرَسَةٌ. If you learn it bare, you will later have to guess whether it is definite.
• Say the sun-letter words out loud while watching the shadda. The shadda is not decoration - it is the pronunciation instruction, and hearing الرَّجُلُ with a long ر is the whole point.
• Use the tongue test to cut the list down: if the consonant is made at the teeth or just behind them - where ل is made - it is solar. Lips, throat and back of the mouth are lunar. Learn ج by heart as the exception: it feels solar today, but it is lunar.
• Remember that ل is solar itself. اللُّغَةُ is not an exception; it is the rule applied to the article’s own letter.
• Stop looking for an Arabic word for “a”. Indefiniteness is a sound at the end of the noun, not a word at the front of it.
• Before writing any noun, run the two-second check: does it begin with الـ? Then it cannot end in tanwīn - and vice versa.
• Ignore for now which case a noun should be in. This lesson only asks you to pair the right definite form with the right indefinite one: البَيْتُ with بَيْتٌ، البَيْتَ with بَيْتًا.
• In ordinary unvocalized text - the pronunciation guide’s own subject - الـ is still plain to see, but the tanwīn vanishes except for the alif of كتابا. Reading definiteness there is mostly reading the front of the word.