In short

A verb whose third root letter is و or ي is called ناقِص, “deficient”, and the name is fair: the weak letter sits at the very end of the word, exactly where the endings go, so it is constantly being squeezed, swallowed or deleted. رَمى، دَعا، مَشى، نَسِيَ، بَقِيَ، أَعْطى، اِشْتَرى - these are not exotic verbs, they are among the most common verbs in the language, and their forms look far shorter than a learner expects. لَمْ يَشْتَرِ is a complete, correct, four-letter word.

The good news is that there are only three patterns, and the whole system is one idea repeated: a long final vowel survives where the ending is a long vowel, and collapses everywhere else. Once you can hear that, رَمَتْ، رَمَوْا، لَنْ يَرْمِيَ، لَمْ يَرْمِ، اِرْمِ stop being separate facts and become one verb seen from five angles. The derived measures add nothing new at all: أَعْطى and اِسْتَدْعى end exactly like رَمى.

The rule

1. The three Form I classes. The dictionary gives you the ماضي and the مُضارِع; that pair, not the root alone, tells you which class you are in.

• و-class: دَعا - يَدْعو (د ع و) “to invite, to call”. Also نَجا - يَنْجو “to escape”, شَكا - يَشْكو “to complain”.
• ي-class: رَمى - يَرْمي (ر م ي) “to throw”; مَشى - يَمْشي “to walk”; بَنى - يَبْني “to build”; جَرى - يَجْري “to run, to happen”.
• a-class (فَعِلَ - يَفْعَلُ): نَسِيَ - يَنْسى “to forget”; لَقِيَ - يَلْقى “to meet”; رَضِيَ - يَرْضى “to be content”; بَقِيَ - يَبْقى “to remain”. Their ماضي has a kasra on the middle radical and their مُضارِع ends in ـى whatever the root letter is.

2. Spelling: ا or ى at the end of the ماضي. In a three-letter verb the final long vowel of the هُوَ form is written ا if the root letter is و (دَعا، نَجا، دَنا) and ى if it is ي (رَمى، مَشى، بَنى، جَرى). That final letter is a reliable clue to the class: a past-tense verb spelled with ا has a و-root and therefore a مُضارِع in ـو. In verbs of more than three letters the ending is written ى: أَعْطى، اِشْتَرى، تَمَنّى، اِسْتَدْعى - unless the letter in front of it is a ي, and then it is a plain ا (أَحْيا، اِسْتَحْيا) - see Hamza, madda and alif maqsura (reading & pronunciation guide).

3. The ماضي: the weak letter survives before a consonant. Endings that begin with a consonant - ـتُ، ـتَ، ـنا، ـنَ - leave the root intact, so you can still see the ي or the و: رَمَيْتُ، رَمَيْنا، رَمَيْنَ / دَعَوْتُ، دَعَوْنا، دَعَوْنَ. Endings that begin with a vowel squeeze it: هِيَ رَمَتْ (not رَمَيَتْ), هُمْ رَمَوْا (not رَمَيوا), هِيَ دَعَتْ، هُمْ دَعَوْا. The هُوَ form has no ending at all and the weak letter has become the long vowel itself: رَمى، دَعا.

The a-class behaves differently and more simply: its ي stays visible in the third person singular - نَسِيَ، نَسِيَتْ - and contracts to a long ي before the consonant endings (نَسيتُ، نَسينا، نَسينَ) and to a long و in the masculine plural (نَسُوا).

4. The مُضارِع المَرْفوع and its invisible ḍamma. يَرْمي، يَدْعو، يَنْسى are indicative forms, but you cannot write a ḍamma on a long vowel. In the terms of The case system: irab and its markers the ending is مُقَدَّر, estimated: it is there grammatically and absent phonetically. For يَرْمي and يَدْعو the reason given is الثِّقَل (heaviness - the vowel is simply too heavy to pronounce there); for يَنْسى it is التَّعَذُّر (impossibility - the alif maqṣūra can carry nothing at all).

5. Homographs to memorise now. In the ي-class, أَنْتِ تَرْمينَ and أَنْتُنَّ تَرْمينَ are identical - and so are أَنْتِ تَنْسَيْنَ and أَنْتُنَّ تَنْسَيْنَ in the a-class. In the و-class, هُمْ يَدْعونَ and هُنَّ يَدْعونَ are identical. Nothing distinguishes them in the indicative - only the surrounding sentence, or a change of mood, tells you which is which: لَنْ تَرْمي (أَنْتِ) against لَنْ تَرْمينَ (أَنْتُنَّ). The feminine plural, as always, is frozen and never changes for mood.

6. The مَنْصوب: a fatḥa is light enough to appear - except on ى. After أَنْ، لَنْ، لِـ، حَتّى the ي and و classes take a visible fatḥa, and the long vowel becomes a plain consonant again:

لَنْ يَرْمِيَ “he will not throw”, لَنْ يَدْعُوَ “he will not invite”.

The a-class cannot do this, because ى takes no vowel: لَنْ يَنْسى looks exactly like the indicative, and the fatḥa is مُقَدَّر. So of the three classes, two show the subjunctive clearly and one hides it completely.

7. The مَجْزوم deletes the weak letter and keeps its short vowel. This is the headline rule of the lesson. After لَمْ، لا النّاهِيَة، لامُ الأَمْرِ and the conditional particles, the final long vowel is not shortened but removed, leaving the preceding consonant with the matching short vowel: ي leaves a kasra, و leaves a ḍamma, ى leaves a fatḥa.

لَمْ يَرْمِ ، لَمْ يَدْعُ ، لَمْ يَنْسَ ، لَمْ يَمْشِ ، لَمْ يَبْقَ.

Grammatically the mood marker is not a sukūn here but حَذْفُ حَرْفِ العِلَّةِ, “the deletion of the weak letter”. A practical consequence: because these forms end in a short vowel and not in a sukūn, they never collide with a following هَمْزَةُ الوَصْلِ. لَمْ يَقُلِ الحَقيقَةَ needs a repair kasra; لَمْ يَرْمِ الكُرَةَ needs nothing, it already has one.

8. Where the ending is a long vowel, nothing is deleted. The five cells that end in ـينَ، ـانِ، ـونَ behave as they do in any verb: they drop the نون and keep the vowel. لَمْ يَرْموا ، لَمْ تَرْمي ، لَمْ يَدْعوا ، لَمْ يَنْسَوْا. And the two feminine plurals never move at all: لَمْ يَرْمينَ، لَمْ يَدْعونَ، لَمْ يَنْسَيْنَ. So the visible collapse belongs to the four suffixless cells أَـ، تَـ، يَـ، نَـ - the same four that mattered in The jussive of the imperfect.

9. The imperative is the jussive with the prefix removed. Take لَمْ يَرْمِ، strip يَـ، prefix a هَمْزَةُ وَصْلٍ vowelled to match the stem: اِرْمِ “throw!”, اُدْعُ “invite!”, اِنْسَ “forget!”, اِمْشِ “walk!”. They are startlingly short - a هَمْزَةُ وَصْلٍ, two consonants and a bare short vowel - and learners routinely refuse to believe them. The prohibition uses the same shape: لا تَرْمِ، لا تَدْعُ، لا تَنْسَ. Feminine and plural addressees keep their long vowel: اِرْمي، اِرْموا، اُدْعي، اُدْعوا، اِنْسَيْ، اِنْسَوْا.

10. The derived measures end identically. Every measure whose مُضارِع ends in ـي inflects like يَرْمي, and the two measures that end in ـى (V تَفَعَّلَ and VI تَفاعَلَ) inflect like يَنْسى. أَعْطى - يُعْطي (IV), اِشْتَرى - يَشْتَري (VIII), اِسْتَدْعى - يَسْتَدْعي (X) all give أَعْطَيْتُ، أَعْطَتْ، أَعْطَوْا and لَمْ يُعْطِ، أَعْطِ; تَمَنّى - يَتَمَنّى (V) gives تَمَنَّيْتُ، تَمَنَّتْ، تَمَنَّوْا and لَمْ يَتَمَنَّ، تَمَنَّ. There is genuinely no new paradigm to learn after Form I.

11. One verb worth flagging: رَأى - يَرى. It is both hamzated and ناقِص, and it loses its hamza in the imperfect: يَرى، تَرى، نَرى, jussive لَمْ يَرَ “he did not see”. Treat it as a memorised item; its ماضي رَأى، رَأَتْ، رَأَوْا is regular for the ي-class.

12. What is left to other lessons. The passive دُعِيَ - يُدْعى and its deputy agent are The passive and the deputy agent. The participles داعٍ and مَدْعُوٌّ, which are where a ناقِص verb becomes a noun, are Derived nouns: participles, place, time and instrument. The behaviour of nouns like قاضٍ - القاضي - قاضِيًا was Diptotes and other declension classes; today is only the verb.

Forms & patterns

The ماضي of all three classes. The third-person dual cells are included; for أَنْتُما the forms are رَمَيْتُما، دَعَوْتُما، نَسيتُما.

Two traps in that table: رَمَيْنا (نَحْنُ) and رَمَيْنَ (هُنَّ) differ only by the final alif, and the dual دَعَوَا differs from the plural دَعَوْا only by the vowel on the و.

The مُضارِع المَرْفوع.

Note أَنْتِ تَدْعينَ against هُمْ/هُنَّ يَدْعونَ: the و-class takes a kasra and a long ي in the feminine singular, because ـو plus ـينَ cannot coexist.

The three moods side by side, third person and the cells where the mood is visible.

The duals follow the same logic and are left out for space: لَنْ يَرْمِيا، لَمْ يَرْمِيا.

The imperative.

Derived measures - the same endings, one row each.

Examples

Common mistakes

• لَمْ يَمْشي -> لَمْ يَمْشِ - the مَجْزوم does not shorten the weak letter, it deletes it. What is left is the kasra that was under the ش all along.
• لَمْ يَدْعو -> لَمْ يَدْعُ - same rule in the و-class. The word is three letters long and that is correct.
• يَرْميونَ -> يَرْمونَ - the plural ـونَ replaces the final ي; it is never added on top of it. Likewise يَنْسَيونَ is يَنْسَوْنَ.
• رَموا -> رَمَوْا - write the sukūn on the و, and remember the alif after it is silent (أَلِفٌ فارِقَةٌ, a purely graphic mark). Reading it as a vowel is the single most audible learner error in this class - the same alif sits in دَعَوْا and نَسُوا.
• لَنْ يَرْمي -> لَنْ يَرْمِيَ - a fatḥa is light enough to be pronounced on the ي, so in the مَنْصوب it must actually be written and said.
• لَنْ يَنْسَ -> لَنْ يَنْسى - the opposite trap: the a-class keeps its ى in the مَنْصوب (the fatḥa is merely estimated). Only لَمْ deletes it.
• دَعى / رَمي -> دَعا / رَمى - the root decides the spelling: و-root gives a final ا، ي-root gives a final ى. Dropping the two dots off ي, or adding them to ى, changes the word.
• نَسَيْتُ -> نَسيتُ - the a-class contracts to a long ي before the ت endings; only the ي-class keeps a diphthong there (رَمَيْتُ).
• دَعَيْتُ -> دَعَوْتُ - د ع و has a و, and the consonant endings show it. If you can say يَدْعو you can predict دَعَوْتُ.
• اِرْمي الكُرَةَ (to a man) -> اِرْمِ الكُرَةَ - اِرْمي with a long ي is the feminine singular imperative. The masculine really does end in a bare kasra.

Tips

• Learn every ناقِص verb as a pair: رَمى - يَرْمي، دَعا - يَدْعو، نَسِيَ - يَنْسى. The class is not predictable from the ماضي alone (compare دَعا with رَمى - the same shape on the page in an unvocalized text).
• Memory hook for the jussive and the imperative: the tail falls off and leaves its vowel behind. ي leaves a kasra, و leaves a ḍamma, ى leaves a fatḥa. Three facts, and they cover لَمْ يَرْمِ، لَمْ يَدْعُ، لَمْ يَنْسَ، اِرْمِ، اُدْعُ، اِنْسَ.
• Drill the هُمْ ماضي out loud - رَمَوْا، دَعَوْا، نَسُوا - because that is where the three classes sound different from each other, and it is the form that most often appears in narrative prose.
• Do not be alarmed by very short words in the press: لَمْ يَبْقَ مِنَ الوَقْتِ شَيْءٌ، لَمْ يَنْتَهِ التَّحْقيقُ، لَمْ يُعْطِ جَوابًا. Each is a normal jussive with its tail deleted.
• When you are unsure of a jussive, escape to ما plus the ماضي: ما رَمى، ما نَسِيَ are correct and buy you thinking time, though لَمْ is the commoner choice in modern prose.
• The homograph pairs (تَرْمينَ for أَنْتِ and أَنْتُنَّ; يَدْعونَ for هُمْ and هُنَّ) are worth a flashcard each. Context resolves them; guessing does not.
• Ignore for now what these verbs do in the passive and as participles. داعٍ، مَدْعُوٌّ، مُشْتَرٍ are coming, and they will make more sense once the active conjugation is automatic.