In short

A three-letter root is not one verb. It is a small factory that can turn out up to ten verbs, each built on a different mould and each meaning something predictably different. From ع-ل-م Arabic makes عَلِمَ “he knew”, عَلَّمَ “he taught”, أَعْلَمَ “he informed”, تَعَلَّمَ “he learned” and اِسْتَعْلَمَ “he inquired”. English needs five unrelated words for that; Arabic needs one root and five shapes. Those shapes are called الأَوْزان, the measures, and Western books number them with Roman numerals I to X.

This lesson is the map, not the territory. You will not conjugate anything here - that is lessons 47 and 53. What you get is the whole grid on one page: which measure looks like what, what each one tends to do to the root’s meaning, and two small diagnostics that let you name a verb’s measure the moment you see it. It is the single highest-value hour in the Arabic verb, because after it every new verb you meet arrives pre-sorted: not “another word to memorise” but “ع-ل-م, Form X”.

The rule

1. الميزانُ الصَّرْفِيُّ: weighing a verb against فعل. Lesson 22 introduced the measuring stick. The root ف-ع-ل is used as a ruler: ف stands for the first root letter, ع for the second, ل for the third, and everything else printed in the mould - a shadda, a long ا, a hamza, a تـ, a نـ, a سـ - belongs to the mould and not to your root. So دَرَّسَ is on the measure فَعَّلَ, اِسْتَقْبَلَ is on the measure اِسْتَفْعَلَ, and اِجْتَمَعَ is on the measure اِفْتَعَلَ. Say the measure out loud after the verb and the shape will start to be visible to you: اِسْتَقْبَلَ اِسْتَفْعَلَ.

2. مُجَرَّد against مَزيد. A verb whose past tense contains nothing but its three root letters and their vowels is مُجَرَّد, “stripped bare”: كَتَبَ، شَرِبَ، كَبُرَ. That is Form I, and it is the only measure with no added material at all. Every other measure is مَزيد, “augmented” - it adds one, two or three letters to the skeleton. The additions are never random. They come from the same small stock lesson 22 taught you to suspect: a doubled middle radical (II), a lengthening ا (III), a hamza (IV), a تـ in front (V, VI), a نـ in front (VII), a ت pushed inside (VIII), a doubled last radical (IX), and اِسْتـ in front (X).

3. Two names for every measure. Arabs name a measure by its own shape - الوَزْنُ فَعَّلَ - or by an ordinal, الوَزْنُ الثّاني. English-language books and dictionaries use Roman numerals, because “Form II” is easier to index than فَعَّلَ. Both name the same thing, and a root-ordered dictionary prints the measures of a root in exactly this order: I, then II, then III, down to X. Learn the numbers as a filing system and the shapes as the reality.

4. What each measure does - a tendency, never a law. The table below gives each measure’s characteristic effect: II intensifies or makes someone do it, III aims the action at another party, IV makes someone do it, V is the reflexive of II, VI is the reciprocal of III, VII is what happens to something with nobody named as the doer, VIII is often a middle voice or simply another lexical verb, IX is colours and bodily defects, X is asking or seeking. These are tendencies that hold often enough to be worth knowing and not often enough to be trusted. اِسْتَخْدَمَ is Form X and means “he used”, not “he sought service”. اِسْتَطاعَ is Form X and means “he was able”. Read the tendency as a first guess that the dictionary either confirms or overrides.

5. One root straight across. The clean case is ع-ل-م, “knowing”, where five measures are in everyday use and their meanings really do follow the tendencies: عَلِمَ “he knew” (I), عَلَّمَ “he taught”, literally “he made someone know” (II), أَعْلَمَ “he informed” (IV), تَعَلَّمَ “he learned”, the reflexive of “teach” (V), اِسْتَعْلَمَ “he inquired”, the seeking of knowledge (X). Notice that ع-ل-م has no ordinary Forms III, VI, VII, VIII or IX in modern use. That is normal. No root fills all ten slots; a typical root has three or four live measures, and the dictionary is the only authority on which. You may recognise a measure, but you may not invent one.

6. First diagnostic: the vowel on حَرْفُ المُضارَعَة. Every imperfect verb starts with a prefix letter - أ، ن، ي، ت. Of the ten measures, that prefix carries a ḍamma in Forms II, III and IV, and a fatḥa in all the others:

• ḍamma: يُدَرِّسُ (II)، يُساعِدُ (III)، يُرْسِلُ (IV).
• fatḥa: يَكْتُبُ (I)، يَتَعَلَّمُ (V)، يَتَعاوَنُ (VI)، يَنْكَسِرُ (VII)، يَجْتَمِعُ (VIII)، يَحْمَرُّ (IX)، يَسْتَقْبِلُ (X).

This is the most useful single fact in the whole system, because it splits the ten measures into three and seven at a glance. Two things sit outside that split. The passive also takes a ḍamma prefix - يُكْتَبُ “it is written” is Form I, not Form IV. The two are told apart by the vowel before the last radical (a fatḥa in the passive: يُكْتَبُ against يُرْسِلُ), and by the fact that the passive of a Form I verb has no visible augment. Lesson 52 handles it properly. And the four-letter measure فَعْلَلَ takes a ḍamma too: تَرْجَمَ ← يُتَرْجِمُ (point 10). So for now, read a ḍamma prefix as “II, III, IV - a four-letter root - or a passive”.

7. Second diagnostic: هَمْزَةُ القَطْعِ against هَمْزَةُ الوَصْلِ. Four measures begin their past tense with an alif, and the alif is not the same alif.

• Form IV begins with هَمْزَةُ القَطْع, written أَ and always pronounced: أَعْلَمَ، أَرْسَلَ، أَغْلَقَ. It is a genuine consonant. In the imperfect it disappears altogether: أَرْسَلَ ← يُرْسِلُ، not يُأَرْسِلُ.
• Forms VII, VIII, IX and X begin with هَمْزَةُ الوَصْل, written as a bare ا and pronounced i- only when the word starts a phrase: اِنْكَسَرَ، اِجْتَمَعَ، اِحْمَرَّ، اِسْتَقْبَلَ. Mid-sentence it is elided - you write the alif but do not pronounce it: وانْكَسَرَ الزُّجاجُ، ثُمَّ اجْتَمَعوا. In the imperfect it vanishes from the writing too: يَنْكَسِرُ، يَجْتَمِعُ، يَسْتَقْبِلُ.

So among the derived measures a past-tense verb starting with أ is Form IV, and one starting with a bare ا is VII, VIII, IX or X. (A hamza-initial Form I verb like أَكَلَ “he ate” or أَخَذَ “he took” also opens with أَ, but there the hamza is a root letter, not a measure marker.) This is a spelling rule with teeth: writing أَسْتَقْبَلَ for اِسْتَقْبَلَ is an error, not a variant, and it is one of the commonest in learner writing.

8. Neither diagnostic survives an unvocalized page. Both clues live in marks that ordinary Arabic does not print. Unvocalized, يدرس is either يَدْرُسُ “he studies” or يُدَرِّسُ “he teaches”; علم is عَلِمَ، عَلَّمَ، عُلِمَ، عِلْمٌ or عَلَمٌ. What you actually read is the consonant skeleton plus the context, exactly as lesson 22 predicted: the letters give you the root, the mould gives you the measure, and the sentence chooses between the moulds that fit. يدرس الأستاذ اللغة العربية في الجامعة is “the professor studies”; يدرس الأستاذ الطلاب اللغة العربية is “the professor teaches”, because a second accusative has appeared and, of the readings this skeleton allows, only the causative measure governs two objects. Note that the subject has to be named for the test to work: يدرس الطلاب اللغة العربية on its own is still ambiguous, since الطلاب can just as well be the nominative subject of يَدْرُسُ. Train the habit now: when a skeleton is ambiguous, ask what the surrounding nouns are doing.

9. Form I’s imperfect vowel is lexical - stop looking for a rule. The augmented measures are regular: once you know a verb is Form II, يُفَعِّلُ is guaranteed. Form I is not. Its imperfect takes a ḍamma, a kasra or a fatḥa on the middle radical, and the choice is a property of the individual verb that you must learn with it: كَتَبَ/يَكْتُبُ، ضَرَبَ/يَضْرِبُ، فَتَحَ/يَفْتَحُ، شَرِبَ/يَشْرَبُ، كَبُرَ/يَكْبُرُ. Two weak tendencies help a little - a past with a kasra (شَرِبَ) usually gives a fatḥa imperfect, and a root containing a throat letter (ح، خ، ع، غ، ه، ء) often takes a fatḥa: فَتَحَ/يَفْتَحُ، ذَهَبَ/يَذْهَبُ. Neither is reliable. Write the imperfect into your notebook every time you meet a new Form I verb.

10. Which measures you actually meet. I, II, V, VIII and X are everywhere in modern prose. III, IV, VI and VII are common. IX is marginal - it exists almost only for colours and physical defects (اِحْمَرَّ “turn red”, اِسْوَدَّ “turn black”, اِعْوَجَّ “become crooked”) and you will meet it mostly in reading. Classical grammars also list measures XI to XV (اِحْمارَّ and friends); they are vanishingly rare in Modern Standard Arabic and you can ignore them entirely. Four-letter roots run on their own small system, فَعْلَلَ - تَرْجَمَ “he translated” - which lesson 48 covers.

11. What this lesson deliberately leaves alone. Conjugating each measure through the persons, the moods and the imperative is lessons 47-53; roots containing ء، و، ي or a doubled radical distort these shapes and get lessons 49-51. The passive is lesson 52, the participles and place nouns lesson 53, and the verbal noun - largely regular for II-X, wildly irregular for I - lesson 54. For now, learn to name a measure and expect a meaning.

Forms & patterns

The ten measures, in the الماضي and the المُضارِع المَرْفوع of هُوَ only. Everything else - the other persons, the moods, the imperative, the مَصْدَر - is left to lessons 47-53 and 59.

The same ten with a live verb in each slot:

One root traced across the system - ع-ل-م. The empty measures are empty in real usage:

Diagnostic 1 - the vowel on the prefix letter:

Diagnostic 2 - which alif starts the past tense:

Form I’s imperfect vowel, which no rule predicts:

What an unvocalized skeleton hides - the measure is a guess until the sentence decides:

Examples

Common mistakes

• دَرَسَ الأُسْتاذُ الطُّلّابَ -> دَرَّسَ الأُسْتاذُ الطُّلّابَ - one shadda is the whole difference between studying something and teaching someone. Read the shadda before you translate.
• يُدْرُسُ الطّالِبُ -> يَدْرُسُ الطّالِبُ - an active Form I never takes a ḍamma on the prefix. The ḍamma belongs to II, III and IV, to the four-letter فَعْلَلَ, and to the passive.
• يُجْتَمِعُ الوُزَراءُ -> يَجْتَمِعُ الوُزَراءُ - Form VIII is outside the ḍamma group; every measure from V to X starts its imperfect with a fatḥa.
• أَسْتَقْبَلَ الرَّئيسُ ضُيوفَهُ -> اِسْتَقْبَلَ الرَّئيسُ ضُيوفَهُ - Form X begins with هَمْزَةُ الوَصْل, a bare alif. Only Form IV is spelled with أ.
• اِعْلَمَ المُديرُ المُوَظَّفينَ -> أَعْلَمَ المُديرُ المُوَظَّفينَ - and the trap in the other direction: Form IV’s hamza is a real consonant, so it must be written and pronounced.
• يُأَرْسِلُ الوَلَدُ الرِّسالَةَ -> يُرْسِلُ الوَلَدُ الرِّسالَةَ - Form IV’s hamza survives only in the past. The imperfect drops it and keeps the ḍamma prefix as its trace.
• يَتَعَلِّمُ الأَطْفالُ -> يَتَعَلَّمُ الأَطْفالُ - Form V keeps a fatḥa on the doubled radical (يَتَفَعَّلُ). The kasra there belongs to Form II, يُعَلِّمُ.
• كَتَبَ ← يَكْتِبُ -> كَتَبَ ← يَكْتُبُ - Form I’s imperfect vowel is a lexical fact, not a derivation. Learn it as part of the word, the way you learn a plural.
• اِنْكَسَرَ الوَلَدُ الزُّجاجَ -> كَسَرَ الوَلَدُ الزُّجاجَ - Form VII names no doer and takes no object. If you want an agent and an object, use Form I.
• اِسْتَخْدَمَ = “he asked for service” -> اِسْتَخْدَمَ = “he used” - the “seeking” of Form X is a tendency in the mould, not a promise in the dictionary. Always confirm the actual meaning.

Tips

• Memorise the two vocalized skeletons فَعَّلَ/يُفَعِّلُ and اِسْتَفْعَلَ/يَسْتَفْعِلُ first. They are among the most productive measures in modern Arabic, and everything else fits around them.
• Chant the prefix rule as a single line - ضَمَّة في الثّاني والثّالِثِ والرّابِعِ، وفَتْحَة في الباقي. It resolves most identification questions in under a second.
• Keep a page in your notebook per root, laid out as ten numbered rows, and fill in a row whenever you meet a new verb. Seeing the gaps is as instructive as seeing the entries.
• Whenever you learn a new verb, record three things and never fewer: the past, the imperfect and the measure number. With Form I, the imperfect vowel is the part you will forget.
• Treat the meaning column as a hypothesis generator, then confirm it in the dictionary. Ignore Form IX until you read a novel, and ignore measures XI-XV forever - they are Classical shapes.
• Practise on headlines, where the measures are dense: اِجْتَمَعَ، اِسْتَقْبَلَ، أَعْلَنَ، اِسْتَأْنَفَ. Name the measure before you look for the meaning.
• Do not try to conjugate yet. Recognition first, production in the next two lessons - the shapes need to be familiar before the paradigms will stick.