In short

الاسْتِثْناء is how Arabic takes something out of a set: “everyone came except one student”. The machinery is small - a particle, إِلّا, plus two nouns that can stand around it - but the case on the excepted noun is not free. It is decided by the shape of the sentence, and there are only three shapes to recognise. Learn to sort the sentence first, and the ending falls out by itself.

The pay-off is bigger than “except”, because the third shape is one of the two standard ways Modern Standard Arabic says only. Alongside the plain adverb فَقَطْ, written Arabic negates the sentence and puts إِلّا in front of the one thing that survives: ما قَرَأْتُ إِلّا كِتابًا = “I read only one book”, literally “I did not read except one book”. That pattern is on every page of a newspaper, and it is the part of this lesson you will use daily.

The rule

1. The three pieces. A full exception sentence has a المُسْتَثْنى مِنْهُ (the set you are taking from), an أَداةُ الاسْتِثْناء (the exception word - إِلّا، غَيْر، سِوى …), and the المُسْتَثْنى (the item taken out, which comes last of the three):

حَضَرَ الطُّلّابُ ← المُسْتَثْنى مِنْهُ · إِلّا ← الأَداةُ · طالِبًا ← المُسْتَثْنى

The only question the lesson answers is what ending that last noun carries. Two yes/no questions settle it: is the set named? and is the sentence negative?

2. تامٌّ مُوجَبٌ - set named, no negation: the accusative is obligatory. This is the classic “except” sentence, and it has exactly one possible ending, no matter what case the set is in:

• حَضَرَ الطُّلّابُ إِلّا طالِبًا. - All the students came except one.
• قَرَأْتُ الكُتُبَ إِلّا كِتابًا. - I read the books except one.
• سَلَّمْتُ عَلى الطُّلّابِ إِلّا طالِبًا. - I greeted the students except one.

Notice the third example: the set is genitive after عَلى, and the excepted noun is still accusative. The excepted noun is not a تابِع here - it does not copy anything. Grammarians call its ending النَّصْبُ عَلى الاسْتِثْناء, “accusative because it is an exception”, and it is the single most reliable rule in this lesson. If the sentence is positive and the set is there, write a fatḥa.

3. تامٌّ مَنْفِيٌّ - set named, sentence negative: بَدَل is preferred, نَصْب is allowed. Once a negator (ما، لَمْ، لَنْ، لا، لَيْسَ) is in front, the excepted noun may instead be read as a بَدَل from the set - the apposition of lesson 58 - and simply copy the set’s case. That reading is the better one, and the one you should write:

• ما حَضَرَ الطُّلّابُ إِلّا طالِبٌ. (بَدَل, nominative like الطُّلّابُ) - also possible: إِلّا طالِبًا.
• ما مَرَرْتُ بِالطُّلّابِ إِلّا طالِبٍ. (بَدَل, genitive like الطُّلّابِ) - also possible: إِلّا طالِبًا.

The choice is only visible when the set is nominative or genitive. If the set is accusative (ما رَأَيْتُ الطُّلّابَ إِلّا طالِبًا) both analyses produce the same fatḥa and nothing is at stake.

4. مُفَرَّغ - no set named, sentence negative: إِلّا becomes transparent. Delete the set, and the verb is left “emptied” (مُفَرَّغ) of its complement. The noun after إِلّا then falls into that empty slot and takes the case its own job in the sentence demands, exactly as if إِلّا were not written. Cover إِلّا with a finger and read the sentence: the ending you get is the right one.

• ما حَضَرَ إِلّا طالِبٌ. - the noun is the فاعِل, so مَرْفوع.
• ما قَرَأْتُ إِلّا كِتابًا. - the noun is the مَفْعولٌ بِهِ, so مَنْصوب.
• ما سَلَّمْتُ إِلّا عَلى طالِبٍ. - the slot is a prepositional phrase, so إِلّا stands in front of the whole phrase and عَلى does the governing: مَجْرور.
• ما هٰذا إِلّا خَبَرٌ قَديمٌ. - the noun is the خَبَر of a nominal sentence, so مَرْفوع.
• لا أَعْمَلُ إِلّا صَباحًا. - an adverbial slot (lesson 31), so مَنْصوب.

This is the shape that means “only”, and it is the centre of the lesson. Everything except the case is fixed: negator + verb + إِلّا + the one surviving element.

5. What licenses a مُفَرَّغ clause. Any negation will do - ما، لَمْ، لَنْ، لا، لَيْسَ (lessons 23 and 37) - and so will a prohibition (لا تَثِقْ إِلّا بِنَفْسِكَ) or a rhetorical question that expects “no one” for an answer (هَلْ يَنْجَحُ إِلّا المُجْتَهِدُ؟). What you cannot do is build one in a plain affirmative sentence: حَضَرَ إِلّا طالِبٌ is not Arabic. For an affirmative “only”, use فَقَطْ, or إِنَّما at the head of the sentence (lesson 68): حَضَرَ طالِبٌ فَقَطْ / إِنَّما حَضَرَ طالِبٌ واحِدٌ.

6. لَيْسَ … إِلّا, and فَقَطْ. لَيْسَ negates like any other negator and licenses the same مُفَرَّغ pattern - لَيْسَ في الغُرْفَةِ إِلّا كُرْسِيٌّ (اسْمُ لَيْسَ، مَرْفوعٌ)، لَسْتُ إِلّا مُوَظَّفًا (خَبَرُ لَيْسَ، مَنْصوبٌ). Separately, لَيْسَ إِلّا is a frozen tag dropped at the end of a sentence to mean “and nothing more”: مُلاحَظَةٌ صَغيرَةٌ، لَيْسَ إِلّا. Nothing follows it and nothing changes shape. فَقَطْ is the plain modern adverb “only”; it is indeclinable, ends in a sukūn, and stands after the element it limits: قَرَأْتُ صَفْحَةً واحِدَةً فَقَطْ. Use one device or the other - stacking ما … إِلّا with فَقَطْ is a beginner’s tautology.

7. غَيْر and سِوى: nouns, not particles. These say the same thing as إِلّا, but they are nouns, so they behave in two layers at once:

• The case on غَيْر / سِوى itself is the case إِلّا’s noun would have taken. Same three sentence types, same decision: حَضَرَ الطُّلّابُ غَيْرَ طالِبٍ (تامٌّ مُوجَبٌ → نَصْبٌ)، ما حَضَرَ الطُّلّابُ غَيْرُ طالِبٍ (تامٌّ مَنْفِيٌّ → بَدَلٌ)، ما حَضَرَ غَيْرُ طالِبٍ (مُفَرَّغٌ، فاعِلٌ → رَفْعٌ)، ما قَرَأْتُ غَيْرَ كِتابٍ (مُفَرَّغٌ، مَفْعولٌ بِهِ → نَصْبٌ)، ما سَلَّمْتُ عَلى غَيْرِ طالِبٍ (مَجْرورٌ بِـعَلى).
• What follows them is مَجْرور, every time a noun follows, because غَيْر/سِوى is the first term of an إِضافَة (lesson 20) and the excepted noun is its مُضافٌ إِلَيْهِ: غَيْرُ طالِبٍ، غَيْرَ الطّالِبِ، سِوى يَوْمَيْنِ.

Two layers, two independent decisions. Learners who freeze غَيْرَ in the accusative get the second layer right and the first one wrong.

8. سِوى shows nothing. سِوى is an اسْمٌ مَقْصورٌ - it ends in ى, so its ending is مُقَدَّر (lesson 41): invisible on the page and inaudible in speech, in all three cases. That makes it comfortable to use, since there is no ending to get wrong, but the noun after it is still a genitive you must vocalize: لَمْ يَبْقَ سِوى يَوْمَيْنِ عَلى الامْتِحانِ (genitive dual, not يَوْمانِ).

9. بِاسْتِثْناءِ، ما عَدا، ما خَلا. Modern prose has three more ways of saying it:

• بِاسْتِثْناءِ + genitive - a compound preposition, and the safest thing to write, because the following noun is a مُضافٌ إِلَيْهِ every single time: حَضَرَ الجَميعُ بِاسْتِثْناءِ طالِبٍ.
• ما عَدا / ما خَلا + accusative - here عَدا and خَلا are verbs and the noun after them is their مَفْعولٌ بِهِ: نَجَحوا جَميعًا ما عَدا واحِدًا. The press variant فيما عَدا works identically: المَكْتَبُ مَفْتوحٌ فيما عَدا الجُمُعَةَ.
• Without ما, عَدا and خَلا may be read either as verbs (accusative) or as prepositions (genitive), so both حَضَرَ الجَميعُ عَدا طالِبًا and عَدا طالِبٍ are correct. Write ما عَدا and the ambiguity disappears.

10. One thing this lesson is not: إِلّا أَنَّ. When إِلّا is followed by أَنَّ it is not excepting anything - it is the connector “however, except that”: الفِكْرَةُ جَيِّدَةٌ، إِلّا أَنَّها مُكْلِفَةٌ. That belongs with the discourse connectors of lesson 66. A different thing again is الاسْتِثْناءُ المُنْقَطِع, where the excepted item is not a member of the set at all (وَصَلَ المُسافِرونَ إِلّا حَقائِبَهُمْ). It is not confined to older Arabic, and its noun is accusative whatever the shape of the sentence - but لٰكِنْ or بَلْ says the same thing more plainly.

Forms & patterns

The whole decision, in one table. Sort the sentence, then read off the ending:

The مُفَرَّغ pattern across the slots - this is the “only” table:

The same three types with إِلّا، غَيْر and سِوى side by side. Read the columns downwards: only the first word of the pair changes case, and the second is always genitive after غَيْر/سِوى.

The remaining exception words, sorted by what they do to the noun after them:

Left out on purpose: حاشا, the third verb of this family, which survives in MSA chiefly in the fixed exclamation حاشا لِلهِ and is not worth a rule; and the classical liberty of putting the excepted noun before the set, which modern prose rarely takes.

Examples

Common mistakes

• حَضَرَ الطُّلّابُ إِلّا طالِبٌ -> حَضَرَ الطُّلّابُ إِلّا طالِبًا - the sentence is positive and the set is named, so the accusative is obligatory. This is the one case with no alternative.
• ما حَضَرَ إِلّا طالِبًا -> ما حَضَرَ إِلّا طالِبٌ - in a مُفَرَّغ clause إِلّا governs nothing. The noun is the subject of حَضَرَ, so it is nominative.
• ما قَرَأْتُ إِلّا كِتابٌ -> ما قَرَأْتُ إِلّا كِتابًا - the same rule pulling the other way: here the noun is the object of قَرَأْتُ, so it is accusative. Read the slot, not the particle.
• ما سَلَّمْتُ إِلّا طالِبًا -> ما سَلَّمْتُ إِلّا عَلى طالِبٍ - إِلّا does not swallow the preposition. سَلَّمَ still needs عَلى (lesson 30), and إِلّا goes in front of the whole phrase.
• حَضَرَ إِلّا طالِبٌ -> ما حَضَرَ إِلّا طالِبٌ / حَضَرَ طالِبٌ فَقَطْ - the مُفَرَّغ pattern needs a negation to empty the slot. Without one, use فَقَطْ.
• ما حَضَرَ غَيْرَ طالِبٍ -> ما حَضَرَ غَيْرُ طالِبٍ - غَيْر is not frozen in the accusative. It occupies the فاعِل slot here, so it is nominative.
• حَضَرَ الطُّلّابُ غَيْرُ طالِبٍ -> حَضَرَ الطُّلّابُ غَيْرَ طالِبٍ - and the same word in a positive complete sentence takes the accusative that إِلّا’s noun would have taken.
• ما حَضَرَ غَيْرُ طالِبٌ -> ما حَضَرَ غَيْرُ طالِبٍ - whatever case غَيْر itself is in, the noun after it is a مُضافٌ إِلَيْهِ and is always genitive.
• لَمْ يَبْقَ سِوى يَوْمانِ -> لَمْ يَبْقَ سِوى يَوْمَيْنِ - سِوى shows no ending of its own, but it is still a مُضاف, and a genitive dual is يَوْمَيْنِ.
• ما عَدا طالِبٍ / بِاسْتِثْناءِ طالِبًا -> ما عَدا طالِبًا / بِاسْتِثْناءِ طالِبٍ - the two are mirror images: ما عَدا is a verb and takes an object, بِاسْتِثْناءِ is a construct and takes a genitive.

Tips

• Two questions, in this order: is the set named? is the sentence negative? Named + positive = fatḥa, no thinking. Named + negative = copy the set’s case. Not named = read the slot.
• For the مُفَرَّغ type, physically cover ما … إِلّا with two fingers and read what is left: حَضَرَ … طالِبٌ، قَرَأْتُ … كِتابًا. The ending that sounds right without إِلّا is the right one.
• Translate ما … إِلّا as “only”, not as “not … except”. The literal reading is what makes learners hesitate over the case; the “only” reading makes the slot obvious.
• Pick one device per sentence. ما … إِلّا or فَقَطْ or بِاسْتِثْناءِ - never two at once.
• غَيْر does two jobs you now know: negating a following noun or adjective (غَيْرُ مُمْكِنٍ, lesson 23) and excepting. The mechanics are identical - it is a مُضاف both times.
• سِوى and بِاسْتِثْناءِ are the low-risk choices when you are writing under pressure: neither asks you to compute an ending on the excepted noun beyond “genitive”.
• Spelling watch, especially in unvocalized text: إِلّا (except, only), إِلى (to), and أَلّا (أَنْ + لا, “that … not”, lesson 34) are three different words with three different shapes.
• Do not over-use the frozen لَيْسَ إِلّا. It is a light conversational tag; in a report, a plain فَقَطْ is better.