In short

Arabic does not have separate words for “to write”, “writer”, “written”, “desk” and “office”. It has the root ك-ت-ب and a set of moulds that turn a verb into a noun: كاتِبٌ the one writing, مَكْتوبٌ the thing written, مَكْتَبٌ the place of writing, مَكْتَبَةٌ the place of books. These noun-moulds are called المُشْتَقّات, “the derived ones”, and there are only four families of them worth learning as a system: the doer, the done-to, the place-or-time, and the tool.

This is the highest-leverage vocabulary lesson in the path. Every measure from lessons 46-48 feeds these moulds automatically, so one new verb hands you three or four new nouns for free - and a strange noun in a newspaper stops being strange the moment you see its mould. The cost is one habit: from Form II upwards the doer and the done-to normally differ by exactly one vowel, and ordinary Arabic prints neither.

The rule

1. Everything starts from the verb. A derived noun is built on a mould, and the mould is chosen by the verb’s measure (Form I or Forms II-X), not by its meaning. So the first question is always the same one lesson 46 taught you to ask: is this verb مُجَرَّد (Form I, three bare radicals) or مَزيد (anything augmented)? Form I has its own idiosyncratic moulds; Forms II-X share one mechanical recipe with the passive of lesson 52.

2. اسْمُ الفاعِلِ from Form I: the mould فاعِل. Slot the three radicals into فاعِل and you have the active participle - the one doing the action: كَتَبَ ← كاتِبٌ، دَرَسَ ← دارِسٌ، عَمِلَ ← عامِلٌ، جَلَسَ ← جالِسٌ. Weak radicals bend the shape in the ways lessons 49-51 predicted:

• الأَجْوَف (weak middle): the و or ي surfaces as a hamza on an alif seat - قالَ ← قائِلٌ، باعَ ← بائِعٌ، نامَ ← نائِمٌ.
• النّاقِص (weak final): the ي is squeezed out and the word ends in ـٍ - دَعا ← داعٍ، قَضى ← قاضٍ، مَشى ← ماشٍ. See rule 10.
• المُضَعَّف (doubled): the two identical radicals merge under a shadda - مَرَّ ← مارٌّ، ضَرَّ ← ضارٌّ.

3. اسْمُ الفاعِلِ from Forms II-X: مُـ plus the imperfect stem plus a كَسْرَة. This is mechanical and almost exception-free - only the أَجْوَف behaves differently, see rule 5. Take the هُوَ imperfect, throw away the prefix letter, put مُـ in its place, and make sure the vowel before the last radical is a كَسْرَة:

يُدَرِّسُ ← مُدَرِّسٌ، يُسافِرُ ← مُسافِرٌ، يُرْسِلُ ← مُرْسِلٌ، يَتَعَلَّمُ ← مُتَعَلِّمٌ، يَتَعاوَنُ ← مُتَعاوِنٌ، يَنْكَسِرُ ← مُنْكَسِرٌ، يَجْتَمِعُ ← مُجْتَمِعٌ، يَسْتَخْدِمُ ← مُسْتَخْدِمٌ. Four-letter roots behave identically: يُتَرْجِمُ ← مُتَرْجِمٌ.

Notice that Forms II, III and IV already had a ḍamma on the imperfect prefix, so for them the change is only a letter; Forms V-X had a fatḥa, so you change both the letter and its vowel.

4. اسْمُ المَفْعولِ from Form I: the mould مَفْعول. The done-to noun of a sound Form I verb is مَفْعول - note the فَتْحَة on the م and the long و: كَتَبَ ← مَكْتوبٌ، عَرَفَ ← مَعْروفٌ، فَتَحَ ← مَفْتوحٌ، شَرِبَ ← مَشْروبٌ. The weak classes again contract:

• الأَجْوَف: the long و collapses into the root vowel - قالَ ← مَقولٌ، باعَ ← مَبيعٌ (a ي root keeps a ي).
• النّاقِص: the final radical doubles under a shadda - دَعا ← مَدْعُوٌّ، رَمى ← مَرْمِيٌّ (و roots in ـوٌّ، ي roots in ـيٌّ).
• المُضَعَّف: مَدَّ ← مَمْدودٌ.

5. اسْمُ المَفْعولِ from Forms II-X: the same مُـ shape with a فَتْحَة. Identical recipe to rule 3, one vowel different: يُدَرِّسُ ← مُدَرَّسٌ، يُشاهِدُ ← مُشاهَدٌ، يُرْسِلُ ← مُرْسَلٌ، يَتَوَقَّعُ ← مُتَوَقَّعٌ، يَحْتَرِمُ ← مُحْتَرَمٌ، يَسْتَخْدِمُ ← مُسْتَخْدَمٌ.

The one class that escapes the vowel contrast is the أَجْوَف of Forms VII and VIII: its middle radical is already a long ا in both moulds, so the two participles fall together - اِخْتارَ / يَخْتارُ gives مُخْتارٌ for “choosing” and “chosen” alike, and اِحْتاجَ gives مُحْتاجٌ for both. Forms IV and X keep them apart, because their active middle stays a long ي: أَقامَ / يُقيمُ ← مُقيمٌ against مُقامٌ, and اِسْتَقامَ / يَسْتَقيمُ ← مُسْتَقيمٌ against مُسْتَقامٌ.

Say this out loud once, because it is the whole lesson: مُدَرِّسٌ and مُدَرَّسٌ differ by one vowel, and unvocalized they are the same four letters, مدرس. So are مُسْتَخْدِمٌ “user” and مُسْتَخْدَمٌ “used”، مُتَّهِمٌ “accuser” and مُتَّهَمٌ “accused”، مُصَدِّرٌ “exporter” and مُصَدَّرٌ “exported”. Real Arabic prints no mark at all, so you decide from the sentence, exactly as the pronunciation guide trained you to: a doer usually has something under it doing something, a done-to usually describes a thing that was acted on.

6. What participles do in a sentence. They are nouns with adjective behaviour. They inflect for gender, number and case like any noun - مُدَرِّسٌ، مُدَرِّسَةٌ، مُدَرِّسونَ، مُدَرِّساتٌ - and they appear in three roles:

• as a plain noun: كاتِبٌ “a writer”, مُهَنْدِسونَ “engineers”, مَشْروبٌ “a drink”.
• as an adjective agreeing with its noun (lesson 8): الطَّريقُ المَقْطوعُ.
• as the predicate of a nominal sentence, where the active participle very often carries an ongoing or an about-to-happen sense: أَنا ذاهِبٌ إِلى السّوقِ “I am going / I am off to the market”, هُمْ قادِمونَ غَدًا “they are coming tomorrow”.

One practical asymmetry: Form I participles of human beings often take a broken plural (كُتّابٌ، عُمّالٌ، طُلّابٌ، قُضاةٌ), while those from Forms II-X take the sound plurals (مُدَرِّسونَ، مُسافِرونَ، مُتَرْجِماتٌ).

7. Not every فاعِل word is a live participle. Many have frozen into ordinary dictionary nouns: عالِمٌ is a scholar or a scientist, not merely “someone who knows”; شاعِرٌ is a poet; طالِبٌ is a student, not “a requester”. The mould tells you where the word came from, the dictionary what it means now. Recognise freely, but do not coin new derived nouns and expect them to be words.

8. اسْما الزَّمانِ والمَكانِ: one mould for two jobs. From Form I the place noun and the time noun share a single shape with a فَتْحَة on the م, and the choice between مَفْعَل and مَفْعِل follows the imperfect vowel:

• imperfect in ـُ or ـَ ← مَفْعَل: يَكْتُبُ ← مَكْتَبٌ، يَطْبُخُ ← مَطْبَخٌ، يَلْعَبُ ← مَلْعَبٌ، يَخْرُجُ ← مَخْرَجٌ.
• imperfect in ـِ ← مَفْعِل: يَجْلِسُ ← مَجْلِسٌ، يَنْزِلُ ← مَنْزِلٌ، and so does the مِثال verb (weak first radical) even where its imperfect is not a kasra: يَعِدُ ← مَوْعِدٌ، يَقِفُ ← مَوْقِفٌ، يَضَعُ ← مَوْضِعٌ.

The same mould often carries a ة: مَدْرَسَةٌ، مَكْتَبَةٌ، مَزْرَعَةٌ، مَحَطَّةٌ. It is not a free variant of the bare shape - where a root has both, they are two separate words (مَكْتَبٌ an office, مَكْتَبَةٌ a library). A handful of very common words are simply irregular and must be learned as vocabulary - مَسْجِدٌ sits on مَفْعِل although يَسْجُدُ has a ḍamma, and so do مَشْرِقٌ، مَغْرِبٌ، مَطْلِعٌ. Weak roots behave as expected: زارَ ← مَزارٌ، جَرى ← مَجْرًى، مَشى ← مَمْشًى، and the everyday المَقْهى “the café” is the same shape.

Two further facts make this family pay. First, from a sound root the plural is predictable: مَفاعِل، a صيغَةُ مُنْتَهى الجُموعِ from lesson 10, hence also a diptote (lesson 41) - no tanwīn, and a fatḥa for the genitive unless it is definite: مَكاتِبُ، مَدارِسُ، مَساجِدُ، في مَكاتِبَ حَديثَةٍ، في المَكاتِبِ. (Place nouns from a hollow root stand outside this and take ـات: مَزارٌ ← مَزاراتٌ.) Second, time and place are the same word - مَوْعِدٌ is the time of an appointment, مَغْرِبٌ is both the west and sunset. Only the sentence decides.

From the derived measures there is no separate mould at all: the place noun is identical to the passive participle, and context alone separates them. مُسْتَشْفًى literally “a place where a cure is sought” is a hospital, مُجْتَمَعٌ is a society, مُخْتَبَرٌ is a laboratory, مُسْتَوْدَعٌ is a warehouse, مُلْتَقًى is a meeting point.

9. اسْمُ الآلَةِ: the tool, with a كَسْرَة on the م. Three classical moulds, all from Form I, and all beginning مِـ:

• مِفْعال: مِفْتاحٌ a key, مِنْشارٌ a saw, مِصْباحٌ a lamp, مِيزانٌ a set of scales.
• مِفْعَل: مِبْرَدٌ a file, مِقَصٌّ a pair of scissors, مِصْعَدٌ a lift.
• مِفْعَلَة: مِكْنَسَةٌ a broom, مِلْعَقَةٌ a spoon, مِسْطَرَةٌ a ruler, مِطْرَقَةٌ a hammer.

Modern machines are named on a newer, fully productive pattern فَعّالَة: غَسّالَةٌ a washing machine, ثَلّاجَةٌ a fridge, سَمّاعَةٌ a receiver. Alongside it sit فَعّال (خَلّاطٌ a blender, سَخّانٌ a heater) and فاعِلَة (رافِعَةٌ a crane, طائِرَةٌ an aeroplane).

Now the contrast that matters: مِـ is the tool, مَـ is the place or the time. مِكْنَسَةٌ is a broom, مَدْرَسَةٌ is a school - the same frame around the root, one vowel apart on the م. A single root can serve both jobs: from غ-س-ل، مَغْسَلَةٌ is the washbasin, the place of washing, and غَسّالَةٌ is the machine. Unvocalized the difference is invisible, which is one more reason these words are learned as vocabulary and not generated on the fly.

10. المَنْقوص participles end in ـٍ when indefinite. قاضٍ، داعٍ، ماشٍ، مُحامٍ lose their final ي in the nominative and the genitive when indefinite and take a plain kasratān; they keep a full ـِيًا in the accusative, and the ي returns whenever the word is definite or the first term of an إِضافَة: القاضي، قاضي المَحْكَمَةِ. This is lesson 41’s declension class met in its commonest habitat, since most مَنْقوص nouns are participles. Their plurals: قُضاةٌ، دُعاةٌ، مُحامونَ.

11. What this lesson leaves alone. A participle can govern an object exactly as its verb does - هُوَ كاتِبٌ رِسالَةً - and that whole topic is lesson 67. The مَصْدَر, the noun of the action itself (كِتابَةٌ، تَدْريسٌ), is lesson 54 - and be warned that one of its shapes, المَصْدَر المِيمِيّ, looks exactly like a place noun (مَطْلَبٌ “a demand”).

Forms & patterns

اسْمُ الفاعِلِ across the measures. The هُوَ imperfect is printed so you can see where the noun comes from:

اسْمُ المَفْعولِ. Form I is مَفْعول; everything else repeats the مُـ shape with a فَتْحَة:

One vowel, two opposite meanings - and nothing at all in unvocalized print:

اسْما الزَّمانِ والمَكانِ from Form I. The plural column is the payoff - from a sound root it is مَفاعِل, and a diptote every time:

From Forms II-X the place noun is the passive participle:

اسْمُ الآلَةِ - the three classical moulds and the modern ones:

The مَنْقوص participle, declined (lesson 41). Only the accusative shows the ي when indefinite:

Examples

Common mistakes

• مُكْتوبٌ عَلى الوَرَقَةِ -> مَكْتوبٌ عَلى الوَرَقَةِ - the Form I passive participle starts with a فَتْحَة on the م. The مُـ prefix belongs to the augmented measures (and to the quadriliteral), never to Form I.
• أَرْسَلَ ← مَرْسولٌ -> أَرْسَلَ ← مُرْسَلٌ - مَفْعول only ever weighs a Form I verb. From an augmented measure you must go through the imperfect: يُرْسِلُ ← مُرْسَلٌ.
• مُدَرَّسُ اللُّغَةِ العَرَبِيَّةِ for “the Arabic teacher” -> مُدَرِّسُ اللُّغَةِ العَرَبِيَّةِ - kasra before the last radical is the doer, fatḥa the done-to.
• مُسافَرٌ إِلى بَيْروتَ -> مُسافِرٌ إِلى بَيْروتَ - سافَرَ is intransitive, so it has no plain passive participle; in ordinary use only مُسافِرٌ occurs.
• دَخَلَ قاضِيٌ إِلى القاعَةِ -> دَخَلَ قاضٍ إِلى القاعَةِ - an indefinite مَنْقوص participle drops the ي and takes ـٍ. It reappears the moment the word is definite: دَخَلَ القاضي.
• مَدْعِيٌّ إِلى الحَفْلِ -> مَدْعُوٌّ إِلى الحَفْلِ - a ناقِص root in و doubles as ـوٌّ (دَعا ← مَدْعُوٌّ); only ي roots give ـيٌّ (رَمى ← مَرْمِيٌّ).
• مِكْتَبُ المُديرِ -> مَكْتَبُ المُديرِ - a kasra on the م announces a tool. The office is a place, so it takes a fatḥa; the broom مِكْنَسَةٌ takes the kasra.
• في مَكاتِبٍ جَديدَةٍ -> في مَكاتِبَ جَديدَةٍ - مَفاعِل is مُنْتَهى الجُموعِ, hence a diptote: no tanwīn, and a fatḥa for the genitive when indefinite.
• الشّاعِرُ = the one who is feeling -> الشّاعِرُ = the poet - dozens of فاعِل words have hardened into ordinary nouns (عالِمٌ، طالِبٌ، تاجِرٌ). Read them as vocabulary first, as participles only if the sentence demands it.

Tips

• Derive from the imperfect, never from the past. For any Form II-X verb the whole procedure is “swap the prefix letter for مُـ, then choose the vowel”: kasra for the doer, fatḥa for the done-to. Two seconds - only a hollow verb needs a second look, since there the two can coincide (يَخْتارُ ← مُخْتارٌ for both).
• Chant the pair يُدَرِّسُ ← مُدَرِّسٌ / مُدَرَّسٌ until it is automatic. Every other measure is the same movement, and this pair carries the hardest fact in the lesson.
• Learn place nouns as vocabulary with a bonus: the singular’s vowel (مَفْعَل or مَفْعِل) is only semi-predictable and worth memorising word by word, but from a sound root the plural مَفاعِل is free every time - and it is a diptote.
• Make the م-prefix your reading reflex: strip the م off an unknown word, read the three letters underneath, and you have the root and a first guess at the meaning.
• Recognise widely, coin narrowly. These moulds explain existing words beautifully; they do not license you to invent a مِفْعال for every gadget.
• Leave two things for later on purpose: how a participle takes its own object (lesson 67), and the مَصْدَر, which is the noun of the action rather than of a person, place or thing (lesson 54).