In short

A hollow verb - الفِعْلُ الأَجْوَفُ, “the verb with a hollow middle” - is one whose second radical is a و or a ي: ق و ل، ب ي ع، ن و م، خ و ف. In the dictionary form that weak letter has already melted into a long alif, so what you actually see is قالَ، باعَ، نامَ، خافَ. The root is invisible on the page, the verb looks a letter short, and none of the endings you drilled on دَرَسَ seem to fit.

They fit perfectly, once you know the single law that governs the whole class: a long vowel cannot stand immediately before a vowelless consonant - the only systematic exception being a following doubled consonant, as in دابَّة. Attach a suffix that begins with a consonant - ـتُ، ـنا، ـنَ - so that the last radical loses its vowel, and the long vowel has to give way to its own short version - قالَ but قُلْتُ. Put the verb into the jussive or the imperative, where the last radical itself takes a sukūn, and it gives way again - لَمْ يَقُلْ، قُلْ. That is the shortening this lesson is named after, and it is the same event in both halves of the verb.

The rule

1. Three Form I types, told apart by the مُضارِع, not by the ماضٍ. The past-tense form is no help: قالَ، باعَ and نامَ all show a bare long alif, and you cannot tell from it whether the middle radical is a و or a ي. The imperfect shows it:

• و-type: قالَ - يَقولُ (ق و ل) “to say”. Also زارَ - يَزورُ، عادَ - يَعودُ، دارَ - يَدورُ، كانَ - يَكونُ.
• ي-type: باعَ - يَبيعُ (ب ي ع) “to sell”. Also سارَ - يَسيرُ، عاشَ - يَعيشُ، زادَ - يَزيدُ، صارَ - يَصيرُ.
• a-type: نامَ - يَنامُ (ن و م) “to sleep”, خافَ - يَخافُ (خ و ف) “to fear”. Also نالَ - يَنالُ (ن ي ل)، plus derived-measure verbs like اِحْتاجَ - يَحْتاجُ.

Look at the third group closely. نامَ and خافَ have a و in the root, and their imperfect is still يَنامُ، يَخافُ with an alif. The type is a fact about the individual verb, exactly like the middle vowel of يَكْتُبُ against يَجْلِسُ in a sound verb. Learn every hollow verb as a pair, قالَ - يَقولُ, and the rest of this lesson is predictable from that pair.

2. The ماضٍ: shorten when the last radical loses its vowel, keep the alif otherwise. The past-tense endings split into two camps. The camp that puts a sukūn on the last radical - ـْتُ، ـْتَ، ـْتِ، ـْتُما، ـْتُمْ، ـْتُنَّ، ـْنا، ـْنَ - forces the shortening. The camp that leaves that radical vowelled, or adds nothing at all - the bare هُوَ form, ـَتْ، ـا، ـَتا، ـوا - leaves the long alif standing:

قالَ، قالَتْ، قالا، قالوا but قُلْتُ، قُلْنا، قُلْنَ.

3. The colour of the short vowel in the ماضٍ. It is not random and it is not always the short twin of the imperfect vowel:

• و-type → ḍamma: قُلْتُ، زُرْتُ، كُنْتُ، دُرْتُ.
• ي-type → kasra: بِعْتُ، سِرْتُ، عِشْتُ، صِرْتُ.
• a-type → kasra, whichever letter the root has: نِمْتُ (و root!)، خِفْتُ (و root!)، نِلْتُ.

So a-type verbs behave one way in the past (kasra) and another in the imperfect (fatḥa). نامَ gives نِمْتُ but لَمْ يَنَمْ, and that split is worth a flashcard of its own.

4. The مُضارِعُ المَرْفوعُ: nothing shortens except before the نون of the feminine plural. The long vowel sits comfortably in يَقولُ، تَقولينَ، يَقولونَ، نَبيعُ، يَنامانِ, because every one of those words ends in a vowel. Only the two frozen feminine-plural cells attach a ـْنَ directly to the last radical and therefore trigger the law: يَقُلْنَ، تَقُلْنَ، يَبِعْنَ، تَبِعْنَ، يَنَمْنَ، يَخَفْنَ. Compare يَقولونَ “they (m.) say” with يَقُلْنَ “they (f.) say” - the same tense, and the length of the stem is the only visible difference.

5. The مَنْصوب keeps the long vowel. Its ending is a fatḥa, a vowel, so no cluster arises and nothing happens: لَنْ يَقولَ، أُريدُ أَنْ أَبيعَ، لَنْ يَنامَ، حَتّى يَعودَ. Learners shorten here out of habit; there is no reason to. The five ن-dropping cells end in a long vowel too and are equally safe: لَنْ يَقولوا، لَنْ تَبيعي.

6. The مَجْزوم shortens - and here the colour follows the imperfect. After لَمْ، لا النّاهِيَة and لامُ الأَمْرِ the last radical takes a sukūn, the law bites, and the long vowel of the imperfect becomes its own short twin:

لَمْ يَقُلْ (و → ḍamma)، لَمْ يَبِعْ (ي → kasra)، لَمْ يَنَمْ and لَمْ يَخَفْ (ا → fatḥa).

The restriction from lesson 35 applies unchanged: only the cells that actually end in a sukūn shorten. The الأَفْعالُ الخَمْسَةُ drop their نون and end in a long vowel, so they keep the stem intact - لَمْ يَقولوا، لَمْ تَبيعي، لَمْ يَناموا. And the feminine plurals were already short in the indicative, so they look identical in all three moods: يَقُلْنَ / لَنْ يَقُلْنَ / لَمْ يَقُلْنَ.

7. The imperative is the jussive with the prefix taken off - so it is short too. Take لَمْ تَقُلْ, remove the تَـ, and you have قُلْ “say!”. Nothing else is needed: unlike اُدْرُسْ, the hollow stem begins with a vowelled consonant, so no هَمْزَةُ الوَصْلِ has to be added. The plural and feminine forms come from the jussive in the same mechanical way, and the cells that end in a long vowel keep it: قُلْ، قولي، قولا، قولوا، قُلْنَ. The same for بِعْ، بيعي، بيعوا and نَمْ، نامي، ناموا and خَفْ، خافي، خافوا. Prohibitions use the jussive directly: لا تَقُلْ هٰذا، لا تَخَفْ.

8. Two sukūns cannot meet. A shortened jussive or imperative ends in a vowelless consonant, and if the next word begins with هَمْزَةُ الوَصْلِ - above all the article - Arabic repairs the collision with a kasra on the verb: قُلِ الحَقيقَةَ، لَمْ يَقُلِ المُديرُ شَيْئًا، لَمْ يَنَمِ الطِّفْلُ. The mood has not changed; only the pronunciation has been smoothed.

9. Derived measures: IV, VII, VIII and X shorten exactly as Form I does. أَقامَ - يُقيمُ، اِخْتارَ - يَخْتارُ، اِسْتَقالَ - يَسْتَقيلُ are all hollow, and all three obey the same law. Two details make them easier than Form I, not harder:

• In the ماضٍ the short vowel is always a fatḥa, whatever the root letter: أَقَمْتُ، أَرَدْتُ، اِخْتَرْتُ، اِحْتَجْتُ، اِسْتَقَلْتُ، اِسْتَطَعْتُ. No three-way choice.
• In the مُضارِع the short vowel is the twin of the long one, as always: يُقيمُ → لَمْ يُقِمْ، أَقِمْ; يَخْتارُ → لَمْ يَخْتَرْ، اِخْتَرْ; يَسْتَقيلُ → لَمْ يَسْتَقِلْ، اِسْتَقِلْ.

10. Measures II, III, V and VI keep the weak letter as a real consonant. In فَعَّلَ and تَفَعَّلَ the middle radical is doubled, and a doubled letter cannot melt: حَوَّلَ - يُحَوِّلُ، عَيَّنَ - يُعَيِّنُ، تَحَوَّلَ - يَتَحَوَّلُ. In فاعَلَ and تَفاعَلَ a long alif already stands in front of it, so it stays a consonant there too: عاوَنَ - يُعاوِنُ، تَعاوَنَ - يَتَعاوَنُ، شاوَرَ - يُشاوِرُ. These verbs are conjugated like any sound verb: حَوَّلْتُ، تَعاوَنّا، لَمْ يَتَحَوَّلْ. A root with a weak middle radical is therefore hollow in some measures and not in others.

11. Reading the skeleton. Hollow verbs are one of the few places where unvocalized Arabic shows you the grammar anyway, because the shortening removes a letter, not just a mark: قال against قلت, يقول against لم يقل, اختار against اخترت. When you meet a bare قل in a text, the word in front of it decides: لم قل is impossible, so لم يقل is a jussive and a lone قل is normally the imperative.

12. Left to later lessons. The passive قيلَ - يُقالُ and بيعَ - يُباعُ belongs to lesson 52 and has its own vowel pattern; the participles قائِلٌ، مَقولٌ، مُقيمٌ، مُخْتارٌ are lesson 53; the verbal nouns قَوْلٌ، بَيْعٌ، نَوْمٌ، إِقامَةٌ، اِخْتِيارٌ are lesson 54. كانَ appears below only as a piece of morphology - what it does to the sentence around it is lessons 32 and 43.

Forms & patterns

الماضي - the past of the three Form I types, with two a-type verbs side by side. Read down the قالَ column and you see the split: the five cells with a long alif are the ones whose ending leaves the ل vowelled, or adds nothing at all.

المُضارِعُ المَرْفوعُ. Only the two feminine plurals are short.

The three moods and the imperative, third person masculine singular. This table is the takeaway of the lesson in one screen.

The imperative across the persons. Only the two cells whose last radical takes a sukūn are short.

Note the trap in the last row: قُلْنَ is both “they (f.) said” and “say! (to women)”, and نَمْنَ “sleep! (to women)” is not the past نِمْنَ “they (f.) slept”. Only the context separates them.

Hollow verbs in the derived measures. The dual and the feminine forms are left out; they follow the Form I pattern exactly.

The measures that are not hollow at all, built on roots with a weak middle radical:

Examples

Common mistakes

• قالْتُ -> قُلْتُ - the suffix ـتُ puts a sukūn on the ل, and a long alif cannot stand in front of it. The alif goes and leaves a ḍamma behind, because قالَ is a و-type verb.
• كانْتُ -> كُنْتُ - the same verb class and the most frequent word it touches. Do not confuse it with كانَتْ “she was”, where the ن keeps its fatḥa and nothing shortens.
• بَعْتُ -> بِعْتُ - the shortening is real, but a ي-type verb leaves a kasra, not a fatḥa. Same for سِرْتُ، عِشْتُ، صِرْتُ.
• نُمْتُ -> نِمْتُ - نامَ has a و in its root, yet its past shortens to a kasra, because it is an a-type verb (يَنامُ). Likewise خِفْتُ، not خُفْتُ.
• لَمْ يَقولْ -> لَمْ يَقُلْ - the جَزْم puts a sukūn on the ل, and a long و immediately before it is impossible. The و disappears from the writing as well.
• لَنْ يَقُلَ -> لَنْ يَقولَ - the مَنْصوب ends in a fatḥa, not a sukūn, so there is no cluster and no reason to shorten. Only the jussive and the imperative are short.
• يَقولْنَ -> يَقُلْنَ - the ـْنَ of the feminine plural attaches to a vowelless consonant, so it shortens the stem in the indicative too, exactly as لَمْ does.
• قولْ -> قُلْ - the imperative is built from the jussive لَمْ تَقُلْ, so it is short from the start. Only the plural and feminine forms keep the long vowel: قولوا، قولي.
• أَقامْتُ -> أَقَمْتُ - the derived measures shorten as well, and in the past their short vowel is always a fatḥa: اِخْتَرْتُ، اِسْتَقَلْتُ، اِحْتَجْتُ.
• قُلْ الحَقيقَةَ -> قُلِ الحَقيقَةَ - two vowelless consonants would meet across the word boundary, so the verb takes a kasra. This is pronunciation, not a change of mood.

Tips

• Store every hollow verb as a triple: قالَ - يَقولُ - قُلْتُ. The first two come from any dictionary; the third is the only thing you have to add, and it fixes the whole past paradigm.
• Two separate colour rules, one per half of the verb. Past: ḍamma for و-type, kasra for everything else. Imperfect side (jussive, imperative, feminine plural): the short twin of the long vowel you already see - يَقولُ → ـُ، يَبيعُ → ـِ، يَنامُ → ـَ.
• Before writing any ending, ask one question: does it leave the last radical without a vowel? ـْتُ، ـْنا، ـْنَ and the sukūn of the jussive do; ـَتْ، ـوا، ـينَ، ـانِ and the fatḥa of the مَنْصوب do not.
• كانَ is worth its own card, drilled as a block: كُنْتُ، كانَ، كانوا، يَكونُ، لَمْ يَكُنْ، كُنْ. It is the hollow verb you will read most often, in texts of every register.
• Practise in minimal pairs and say them aloud: قالَ / قُلْتُ، يَقولونَ / يَقُلْنَ، لَنْ يَقولَ / لَمْ يَقُلْ. The difference is one beat of length, and your ear learns it faster than your eye.
• When a hollow verb blocks you in speech, escape into the past with ما: ما زارَنا is correct and buys you a second while لَمْ يَزُرْنا settles.
• Ignore for now: the passive قيلَ، the participle قائِلٌ and the verbal noun قَوْلٌ all reshape the weak radical again, and each has its own lesson coming. Master the active paradigm first.