In short

Lesson 40 gave you the standard machine: ـٌ in the nominative, ـً in the accusative, ـٍ in the genitive, with the tanwīn disappearing as soon as الـ arrives. A large and entirely ordinary set of nouns does not obey that machine. Some refuse tanwīn altogether and answer the genitive with a fatḥa: مَساجِدُ، أَحْمَدُ، صَحْراءُ. Others end in a long vowel that cannot carry a case mark at all, so the three cases look identical: فَتًى، مُسْتَشْفًى. A third group drops its final ي while indefinite and grows it back in the accusative: قاضٍ، قاضِيًا.

None of this is rare vocabulary. مَدارِس، مَفاتيح، مُسْتَشْفى، مُحامٍ، أَصْدِقاء، أَحْمَر are words you have been using since A1, and every one of them is irregular in precisely the way described here. Each class has one short rule, and naming them is most of the work: once you can see them coming, the classic errors - فِي مَدارِسٍ، إِلى لَنْدَنِ، مُحامي - stop happening.

The rule

1. المَمْنوعُ مِنَ الصَّرْفِ means “barred from tanwīn”. الصَّرْف here is the tanwīn itself, so the name says exactly what happens: the noun never takes ـٌ ـً ـٍ. And because it has lost the tanwīn, it also loses one of its three endings. Instead of ḍamma / fatḥa / kasra it has only two vowels: a ḍamma for the nominative, and a fatḥa for both the accusative and the genitive.

جاءَ أَحْمَدُ (nominative) - رَأَيْتُ أَحْمَدَ (accusative) - سَلَّمْتُ عَلى أَحْمَدَ (genitive). Not عَلى أَحْمَدٍ, not عَلى أَحْمَدِ.

2. The fatḥa in the genitive is the whole difficulty. This is one of the عَلاماتٌ فَرْعِيَّةٌ you met in lesson 40: a substitute mark standing in for the expected one. The preposition still puts the noun in the genitive; the genitive simply shows up wearing a fatḥa. Learners rarely get the nominative or the accusative wrong here, because those look normal. They get the genitive wrong, because everything they have learnt says “after a preposition, kasra”. Say the phrase سافَرْتُ إِلى لَنْدَنَ out loud twenty times until the fatḥa after إِلى stops feeling like a mistake.

Watch what happens to an accompanying adjective, which is usually an ordinary noun and keeps its normal ending: دَرَسَتْ فِي مَدارِسَ كَبيرَةٍ. The noun takes the fatḥa without tanwīn, the adjective takes the kasra with tanwīn - side by side in the same phrase, both correct.

3. The reversal almost everyone misses: الـ and إِضافَة give the kasra back. A diptote is only “defective” while it stands alone and indefinite. Attach the article, or make it the first term of an إِضافَة, and the genitive returns to a plain kasra:

• فِي مَدارِسَ كَبيرَةٍ - indefinite, alone → fatḥa.
• فِي المَدارِسِ الكَبيرَةِ - definite with الـ → kasra.
• فِي مَدارِسِ المَدينَةِ - first term of an إِضافَة → kasra.

The tanwīn never comes back - الـ and an إِضافَة exclude tanwīn anyway (lessons 4 and 20), so there is nothing to restore. Only the vowel changes. A useful way to hold it: the ban on tanwīn is permanent, the fatḥa-for-kasra swap is temporary, and it switches off the moment the word becomes definite.

4. The categories. There are seven you actually need. They are listed in the table below with one example each; here is what to notice about them.

أَعْلامٌ أَعْجَمِيَّةٌ - foreign proper names: إِبْراهيمُ، يوسُفُ، باريسُ، لَنْدَنُ، بَرْلينُ. مُؤَنَّثٌ - feminine proper names, whether or not they end in ة: فاطِمَةُ، زَيْنَبُ، سُعادُ، مَكَّةُ. وَزْنُ أَفْعَلَ - the pattern itself, whichever job it is doing: the name أَحْمَدُ, the elative أَكْبَرُ of lesson 28, the colour adjective أَحْمَرُ. الأَلِفُ والنّونُ الزّائِدَتانِ - words ending in ـانُ: the name عُثْمانُ, the adjective عَطْشانُ. وَزْنُ فُعَلَ - a small closed set of names: عُمَرُ، زُحَلُ، قُزَحُ. صيغَةُ مُنْتَهَى الجُموعِ - the broken-plural shape مَفاعِلُ / مَفاعيلُ: مَساجِدُ، مَكاتِبُ، تَجارِبُ، مَفاتيحُ، عَصافيرُ، مَصابيحُ. أَلِفُ التَّأْنيثِ المَمْدودَةُ - the ending ـاءُ used as a feminine marker: صَحْراءُ، حَمْراءُ، أَصْدِقاءُ، أَطِبّاءُ.

By far the most frequent of these in running text is مُنْتَهَى الجُموع, because it covers hundreds of everyday plurals. Recognise it by shape rather than by memorising members: a broken plural with an alif followed by two letters (مَساجِدُ، دَراهِمُ) or by three with a long vowel in the middle (مَفاتيحُ، قَناديلُ) is a diptote. How those plurals are built is lesson 10; how they decline is here.

5. The near-misses. Three traps sit next to the categories above.

• A three-letter foreign name with a sukūn in the middle is not a diptote: نوحٌ، لوطٌ decline normally. So does the feminine name هِنْدٌ, where both treatments are permitted and the tanwīn is the ordinary choice.
• An adjective in ـانُ is a diptote only when its feminine is فَعْلى: عَطْشانُ / عَطْشى. Where the feminine is فَعْلانَةٌ, the word is regular: نَدْمانٌ / نَدْمانَةٌ. A name in ـانُ - عُثْمانُ، مَرْوانُ - is a diptote whatever its feminine would be, because being a name is already half the reason.
• The same split runs through أَفْعَلُ. An adjective on that pattern is a diptote only when its feminine is فَعْلاءُ or فُعْلى: أَحْمَرُ / حَمْراءُ, أَكْبَرُ / كُبْرى. أَرْمَلٌ, whose feminine is أَرْمَلَةٌ, is an ordinary noun and takes tanwīn. The name أَحْمَدُ, by contrast, is a diptote in any event.

6. الاسْمُ المَقْصورُ: an ending you cannot hear. A مَقْصور noun ends in a fixed alif, written ى or ا: فَتًى، مُسْتَشْفًى، مَقْهًى، مَعْنًى، عَصًا، هُدًى، ذِكْرى، كُبْرى. That alif cannot carry a vowel, so the case ending is مُقَدَّر - assumed, not written. The consequence is the friendliest rule in Arabic grammar: the word never changes.

هٰذا مُسْتَشْفًى كَبيرٌ - دَخَلْتُ مُسْتَشْفًى كَبيرًا - عَمِلْتُ في مُسْتَشْفًى كَبيرٍ. One shape in all three cases. Only the adjective moves, and it moves normally.

Indefinite, the tanwīn is written on the letter before the alif, and it is always the tanwīn fatḥ: فَتًى، مُسْتَشْفًى، مَقْهًى. Definite, nothing at all is written: الفَتى، المُسْتَشْفى، المَقْهى. Two words in the list above never take that tanwīn at all: in ذِكْرى and كُبْرى the alif is the feminine ending أَلِفُ التَّأْنيثِ المَقْصورَةُ, which makes them diptotes as well as مَقْصور.

7. الاسْمُ المَنْقوصُ: the ي that comes and goes. A مَنْقوص noun ends in an unstressed ي after a kasra: القاضي، المُحامي، الوادي، النّادي، المُشْتَري، الماضي. Definite, the ي stays and the case ending is again مُقَدَّر in the nominative and the genitive, but visible in the accusative:

حَضَرَ القاضي - رَأَيْتُ القاضِيَ - ذَهَبْتُ إِلَى القاضي.

Indefinite, something more dramatic happens. In the nominative and the genitive the ي drops out of the written word and its place is taken by a tanwīn kasr on the letter before it: قاضٍ، مُحامٍ، وادٍ، نادٍ. In the accusative the ي returns and takes an ordinary tanwīn fatḥ: قاضِيًا، مُحامِيًا، وادِيًا. So the indefinite مَنْقوص has two shapes, and the odd-looking short one covers two cases out of three.

As first term of an إِضافَة the ي is kept and no tanwīn appears: قاضي المَدينَةِ، مُحامي الشَّرِكَةِ.

8. الاسْمُ المَمْدودُ: usually nothing special. A مَمْدود noun ends in a hamza after a long alif: بِناءٌ، سَماءٌ، دَواءٌ، شِتاءٌ. The hamza is a consonant, it holds a vowel perfectly well, and these nouns decline exactly like كِتابٌ: بِناءٌ / بِناءً / بِناءٍ. Say the whole set aloud once and you are done.

The one thing to check is why the word ends in اء. If the hamza is the feminine marker - أَلِفُ التَّأْنيثِ المَمْدودَةُ - the noun is a diptote, which is category seven above: صَحْراءُ، حَمْراءُ، أَصْدِقاءُ، أَطِبّاءُ، أَصْفِياءُ. If it is not (سَماءٌ from the root س م و, بِناءٌ from ب ن ي, دَواءٌ from د و ي), the noun is ordinary. Two words that look identical in shape can therefore behave differently: فِي صَحْراءَ واسِعَةٍ but فِي سَماءٍ صافِيَةٍ.

9. What this lesson leaves alone. How broken plurals are formed - which noun gives مَفاتيح and which gives كُتُب - is lesson 10; only their declension is treated here. The basic case system, the names of the cases and the standard endings are lesson 40 and are assumed. The dual and the sound masculine plural, which also use substitute markers, belong to lesson 40 as well. And the rarest corners of the diptote list - compound place names of the type حَضْرَمَوْتُ, and the licence poetry takes to add tanwīn to a diptote for the metre - are not part of the modern standard you need.

Forms & patterns

The two-ending declension, with the الـ and إِضافَة reversal in the last two columns. Read the bottom row across - that is the entire lesson in one line:

The seven categories, one example each, shown in the genitive because that is where they betray themselves:

The three “hidden ending” classes. In the مَقْصور and مَنْقوص rows the bolded cells are the only ones where anything visibly moves; the two مَمْدود rows at the bottom decline in plain sight:

Words that look like diptotes and are not. Each takes full tanwīn:

Examples

Common mistakes

• فِي مَدارِسٍ كَبيرَةٍ -> فِي مَدارِسَ كَبيرَةٍ - مَدارِس is صيغَةُ مُنْتَهَى الجُموعِ, so it refuses tanwīn and shows the genitive with a fatḥa. The adjective keeps its normal kasra tanwīn.
• إِلى لَنْدَنِ -> إِلى لَنْدَنَ - a foreign proper name is a diptote; the preposition still governs the genitive, but the mark for it is a fatḥa.
• فِي المَدارِسَ -> فِي المَدارِسِ - the fatḥa is only for the bare indefinite word. Once الـ is attached the ordinary kasra comes back.
• فِي مَدارِسَ المَدينَةِ -> فِي مَدارِسِ المَدينَةِ - the same reversal for an إِضافَة: as first term the noun takes a plain kasra.
• سَلَّمْتُ عَلى أَحْمَدٍ -> سَلَّمْتُ عَلى أَحْمَدَ - أَحْمَد is on the pattern أَفْعَل. No tanwīn ever, in any case.
• سَيّارَةٌ حَمْراءٌ -> سَيّارَةٌ حَمْراءُ - the ـاء of حَمْراء is the feminine marker, so the adjective is a diptote even though the noun in front of it is not.
• مُحامي -> مُحامٍ - an indefinite مَنْقوص loses its ي in the nominative and the genitive; a tanwīn kasr on the preceding letter is all that is left of it.
• رَأَيْتُ مُحامٍ -> رَأَيْتُ مُحامِيًا - in the accusative the ي comes back and takes a normal tanwīn fatḥ. This is the only case where the indefinite مَنْقوص is written in full.
• فَتاً -> فَتًى - the مَقْصور is written with ى and the tanwīn sits on the letter before it, not on the alif. The word is identical in all three cases, so there is nothing to add for the accusative either: دَخَلْتُ مُسْتَشْفًا is likewise دَخَلْتُ مُسْتَشْفًى.
• فِي بِناءَ قَديمٍ -> فِي بِناءٍ قَديمٍ - بِناء is not a diptote. Its hamza is not a feminine ending, so the word declines like any other noun.

Tips

• Learn the categories as a checklist you run only when the word is indefinite and you are about to say a kasra. That is the single moment where the whole topic matters.
• The shape test for the commonest class is faster than any list: broken plural, alif, then two letters (مَكاتِبُ) or three with a long vowel between them (مَصابيحُ) → diptote.
• Proper names are the other half of the everyday load. Run the categories over them: foreign (باريسُ، لَنْدَنُ، إِبْراهيمُ), feminine (فاطِمَةُ، زَيْنَبُ، مَكَّةُ), أَفْعَل (أَحْمَدُ), ـان (عُثْمانُ), فُعَل (عُمَرُ) → diptote. An Arabic masculine name that fits none of them is an ordinary noun and takes tanwīn: مُحَمَّدٌ، خالِدٌ، سَعيدٌ، زَيْدٌ.
• Say مَعَ أَصْدِقاءَ، فِي صَحْراءَ، إِلى باريسَ as fixed blocks. The fatḥa after a preposition has to become a sound you expect, not a rule you compute.
• For the مَقْصور, celebrate: فَتًى، مُسْتَشْفًى، مَقْهًى، مَعْنًى never change. Put them in your sentences freely and spend the attention elsewhere.
• For the مَنْقوص, remember the trade: the ي and the tanwīn kasr are the same thing wearing different clothes. Where one is present the other is absent - قاضٍ / القاضي - and the accusative is the exception that shows both the ي and a tanwīn.
• In unvocalized text almost all of this is invisible, with two loud exceptions you can actually see while reading: the missing ي of محام / قاض, and the ى of فتى. Those are worth recognising on sight; the rest matters when you write and speak.
• Ignore the classical fine print for now - diptotes that regain tanwīn in poetry, and plurals that are diptote and مَنْقوص at once (لَيالٍ، أَراضٍ، جَوارٍ). Meet the second group as vocabulary and copy the forms you see.