In short

The imperfect has three moods, and you already own two. The plain مَرْفوع يَدْرُسُ carries a ḍamma; the مَنْصوب يَدْرُسَ carries a fatḥa and appears after أَنْ، لَنْ، لِـ، حَتّى. The third is the مَجْزوم, and its ending is the absence of an ending - a sukūn: يَدْرُسْ. Like the مَنْصوب, it is never your choice. A particle stands in front of the verb and the ending changes; that is the whole mechanism.

The particle to learn today is لَمْ, and it does something that feels upside-down: it puts a present-tense form in front of you and negates the past. لَمْ يَدْرُسْ is “he did not study”, word for word “not-yet he-study”. The same مَجْزوم endings are the machinery behind commands and prohibitions in the next lesson, so the drilling you do here pays for itself twice.

The rule

1. The جَزْم ending: a sukūn where the ḍamma was. Take the five cells of the imperfect that end in a bare consonant - the أَـ، تَـ، يَـ، نَـ forms - and strip the vowel off the last letter:

أَدْرُسُ → أَدْرُسْ، تَدْرُسُ → تَدْرُسْ، يَدْرُسُ → يَدْرُسْ، نَدْرُسُ → نَدْرُسْ.

That is four distinct shapes covering five cells, because تَدْرُسْ serves both أَنْتَ and هِيَ. The sukūn is the mark of the mood, exactly as the fatḥa was the mark of the مَنْصوب.

2. The الأَفْعالُ الخَمْسَةُ drop their ن - the same drop you already learnt. The five forms that end in a نون - تَدْرُسينَ، تَدْرُسانِ، يَدْرُسانِ، تَدْرُسونَ، يَدْرُسونَ - lose that نون in the مَجْزوم and end in the long vowel alone: تَدْرُسي، تَدْرُسا، يَدْرُسا، تَدْرُسوا، يَدْرُسوا. This is not a new rule. It is the identical drop you met after أَنْ and لَنْ in lesson 33, and the resulting forms are letter-for-letter the same. The wāw of the plural still gets its silent alif of support: يَدْرُسوا، لَمْ يَذْهَبوا.

3. The two feminine plurals never move. تَدْرُسْنَ and يَدْرُسْنَ are مَبْنِيّ - built, frozen. They are the same in all three moods: تَدْرُسُ / تَدْرُسَ / تَدْرُسْ change, but يَدْرُسْنَ is يَدْرُسْنَ after لَمْ, after أَنْ, and on its own. Two cells of thirteen that you can stop worrying about.

4. Say it once and remember it: مَنْصوب and مَجْزوم differ only in the cells that take no suffix. Put the paradigms side by side and eight of the thirteen cells are identical - the six cells filled by the ن-dropping الأَفْعالُ الخَمْسَةُ and the two frozen ones. In a sound verb the entire visible difference between “that he study” and “he did not study” is fatḥa versus sukūn on أَدْرُسَ/أَدْرُسْ، تَدْرُسَ/تَدْرُسْ، يَدْرُسَ/يَدْرُسْ، نَدْرُسَ/نَدْرُسْ - four shapes covering five cells. Learn the جَزْم as “the مَنْصوب with the fatḥa knocked off the four”, and keep in mind that in a weak verb the sukūn does more than that, as point 7 shows.

5. لَمْ negates the past, with an imperfect verb. لَمْ يَدْرُسْ ≈ ما دَرَسَ - the same basic meaning, “he did not study”. Arabic has two ways of saying it and this is the one that dominates modern written and formal-spoken usage: لَمْ plus a مَجْزوم. Two consequences follow immediately. لَمْ can never be followed by a past-tense verb (لَمْ دَرَسَ is not Arabic), and the imperfect after it is not present time at all - the pastness lives in the particle. If you want the past-tense form of the verb, you must switch particles: ما دَرَسَ, which is lesson 37 along with لَنْ.

6. Two more particles that cause جَزْم - named now, drilled later. لا النّاهِيَة, the “don’t” of prohibition: لا تَدْرُسْ “don’t study” (as opposed to لا تَدْرُسُ “you do not study” - the same لا in writing, and only the ending tells you which). And لامُ الأَمْرِ, the lām that orders a third person: لِيَدْرُسْ “let him study”. Both are lesson 36, together with the imperative proper. Register the shape now, so that when you meet لا تَنْسَ you recognise a jussive form rather than an irregular verb. One more, لَمّا “not yet” (لَمّا يَصِلْ “he has not arrived yet”), also takes the جَزْم and waits for lesson 37.

7. Weak verbs: the sukūn cannot land on a long vowel, so the vowel goes instead. This is where the mood becomes loud, and it is the real content of this lesson. A sukūn means “no vowel here”, and a final ي، و، ا is a vowel - the two cannot coexist. So:

• Final-weak (النّاقِص): the weak letter is deleted and its short vowel is left standing. يَمْشي → لَمْ يَمْشِ; يَدْعو → لَمْ يَدْعُ; يَنْسى → لَمْ يَنْسَ. Nothing was added; a letter was removed. The verb now ends in a bare consonant with a kasra, a ḍamma or a fatḥa on it, which is why these forms look so short and startle beginners.
• Hollow (الأَجْوَف): the long vowel in the middle shortens, because the final consonant now carries a sukūn and Arabic will not tolerate a long vowel immediately before it. يَقولُ → لَمْ يَقُلْ; يَبيعُ → لَمْ يَبِعْ; يَكونُ → لَمْ يَكُنْ; يَخافُ → لَمْ يَخَفْ. The short vowel that appears is the short version of the long one: و → ḍamma, ي → kasra, ا → fatḥa.
• Doubled (المُضَعَّف): three forms are all correct. لَمْ يَرُدَّ (fatḥa, the lightest and the most common), لَمْ يَرُدِّ (kasra, by the general rule below), لَمْ يَرْدُدْ (the shadda undone, the doubled letters written out, the sukūn placed as on a sound verb). Use the first; recognise the other two.

8. But only where the ending is a sukūn. The shortening in point 7 is caused by the sukūn, so the five ن-dropping forms - which end in a long vowel, not a sukūn - keep the verb intact: لَمْ يَقولوا “they did not say”, لَمْ تَبيعي “you did not sell”, لَمْ يَمْشوا “they did not walk”. Compare لَمْ يَقُلْ with لَمْ يَقولوا and you can see the two halves of the rule in one pair. Before the نون of the feminine plural the stem is short for the same reason it always is: لَمْ يَقُلْنَ، لَمْ يَرْدُدْنَ.

9. Two sukūns cannot meet: اِلْتِقاءُ السّاكِنَيْنِ. A jussive ending in a sukūn, followed by a word beginning with هَمْزَةُ الوَصْلِ - above all the article الـ - would give you two vowelless consonants in a row. Arabic breaks the collision by putting a kasra on the verb:

لَمْ يَقُلْ + الحَقيقَةَ → لَمْ يَقُلِ الحَقيقَةَ. لَمْ يَكُنْ + الطَّقْسُ → لَمْ يَكُنِ الطَّقْسُ. لَمْ يَدْرُسْ + الوَلَدُ → لَمْ يَدْرُسِ الوَلَدُ.

The kasra is a pronunciation repair, not a change of mood: the verb is still مَجْزوم. You have seen the same repair on مِنْ before the article - مِنَ البَيْتِ - which takes a fatḥa instead; that one is a lexical exception you learn as a word, while verbs take the kasra by rule.

10. In unvocalized text and in pause, the sukūn is invisible. Written يقل and يقول are two different skeletons and you can see the difference; written يدرس is مَرْفوع, مَنْصوب and مَجْزوم at once, and only لَمْ in front of it tells you which. In الوَقْف - stopping at the end of a phrase - the final ḍamma is dropped anyway, so مَرْفوع and مَجْزوم sound identical there too. The practical conclusion: the weak verbs and the five ن-dropping forms are where the mood is actually visible, in speech and in print alike. Those are the ones to master. لَمْ يَقُلْ versus لَمْ يَقولوا, لَمْ يَمْشِ versus لَمْ يَمْشوا - a reader hears the mood there and nowhere else.

11. What is deliberately left out. The imperative فِعْلُ الأَمْرِ (اُدْرُسْ), the prohibition لا تَدْرُسْ and the vocative are lesson 36 - they are built on today’s endings, which is exactly why today comes first. ما and لَنْ, and the full comparison of past and future negation, are lesson 37. The conditional particles إِنْ and مَنْ, which put two verbs into the jussive at once, are lesson 56. And the weak verb classes get their own systematic treatment in lessons 50 and 51; the table below is the working subset you need to read a newspaper sentence with لَمْ in it.

Forms & patterns

The three moods of دَرَسَ - يَدْرُسُ, all thirteen cells. Read it across: the mood is only ever visible at the very end of the word.

The last two columns are identical except where the form takes no suffix - the أَـ، تَـ، يَـ، نَـ shapes, four of them filling five cells. Cover the middle column and you have the paradigm of lesson 33 back.

Weak and doubled verbs, third person masculine singular - the forms where the جَزْم is audible:

The same two verbs across the persons, so you can see the shortening switch itself off wherever the word ends in a long vowel rather than a sukūn:

The three particles that cause جَزْم on a single verb. Only the first is yours to use today:

The bottom two rows are lesson 36 and are here for recognition only; the verb form in them is the same مَجْزوم you have just tabled.

Examples

Common mistakes

• لَمْ يَدْرُسُ -> لَمْ يَدْرُسْ - لَمْ is a jussive particle, so the ḍamma of the indicative cannot survive it. Sukūn, always.
• لَمْ دَرَسَ -> لَمْ يَدْرُسْ - لَمْ takes an imperfect verb and turns it into a past meaning. It can never sit in front of a past-tense form; if you want دَرَسَ, the negator is ما.
• لَمْ يَدْرُسَ -> لَمْ يَدْرُسْ - the fatḥa is the مَنْصوب, which belongs to أَنْ، لَنْ، لِـ، حَتّى. Different particle, different ending.
• لَمْ يَذْهَبونَ -> لَمْ يَذْهَبوا - the الأَفْعالُ الخَمْسَةُ drop their ن in the مَجْزوم exactly as they do in the مَنْصوب, and the silent alif is written after the wāw.
• لَمْ تَدْرُسينَ -> لَمْ تَدْرُسي - the same drop in the feminine singular. What is left is the long ي on its own.
• لَمْ يَقولْ -> لَمْ يَقُلْ - a sukūn cannot follow a long vowel, so the و of the hollow verb shortens to a ḍamma. The written و disappears with it.
• لَمْ يَكونْ -> لَمْ يَكُنْ - the same verb type, and one of the most frequent jussives in the language. Learn لَمْ يَكُنْ as a block.
• لَمْ يَمْشي -> لَمْ يَمْشِ - in a final-weak verb the weak letter is deleted altogether and only its short vowel stays behind. The word really is that short.
• لَمْ يَنْسى -> لَمْ يَنْسَ - identical rule with a final ى: the alif maqṣūra goes, the fatḥa remains.
• لَمْ يَقُلْ الحَقيقَةَ -> لَمْ يَقُلِ الحَقيقَةَ - the sukūn of the verb collides with the hamzat waṣl of الـ, and Arabic resolves it with a kasra on the verb. The mood has not changed.

Tips

• Drill it as a transformation, not as a new paradigm: say the مَنْصوب form, then knock the fatḥa off. أَدْرُسَ → أَدْرُسْ. Eight of the thirteen cells need no transformation at all.
• Do not waste practice on sound verbs like دَرَسَ and كَتَبَ - in the endingless cells their jussive is invisible in ordinary text. Spend the time on قالَ، كانَ، مَشى، نَسِيَ، رَأى، دَعا, where the form visibly collapses and where a wrong answer is heard immediately.
• Memory hook for the weak verbs: the jussive is the short form. Whatever the verb has that can be shortened - a final ي/و/ى, a medial long vowel - the jussive takes it away, but only in the cells that end in a sukūn.
• لَمْ يَكُنْ deserves its own flashcard. It carries “there was no…”, “he/it was not…” and, with a following participle or imperfect, most past negation in the press.
• When you are unsure of a weak jussive, rephrase with ما plus the past: ما مَشى is a perfectly good sentence and buys you time while لَمْ يَمْشِ settles.
• Reading aloud, watch for the article after a jussive and slide into the kasra: لَمْ يَقُلِ الحَقيقَةَ, not a hard stop between the two words.
• With رَدَّ and its fellow doubled verbs, pick لَمْ يَرُدَّ and stay with it. The other two forms are correct but you only need to recognise them.
• Ignore for now what else the jussive does. In conditional sentences it appears twice in one sentence with no negation anywhere in sight; that is lesson 56, and it will make sense once the endings are automatic.