As for number, nouns الأَسْمَاء and verbs الأَفْعَال in Arabic have three forms: singular مُفْرَد, dual مُثَنَّى, and plural جَمْع. The singular refers to a single entity, the dual refers to two entities, and the plural refers to three or more. Forming the dual and plural forms from the singular is usually straightforward. This is done by adding the number suffix to the end of the singular form. This post is about how dual nouns and verbs are formed and how their formation changes when they occur in different positions of the sentence.

Dual in Arabic is marked by a suffix called أَلِفُ الاِثْنَيْن alif al-ithnayn ‘the alif of dual’. It is simply a plain alif ـا that is added to the end of singular form of pronouns, nouns, and verbs. In the case of nouns and verbs, ن is normally added after the alif when they are in the nominative case and indicative mood, respectively. This ن gets dropped in the other cases/moods.

Pronominal Duals:

The first person singular أَنَا has no dual form in Arabic. Both second person singular masculine and feminine have the same dual form, that is أَنْتُمَا. The additional suffix, which is ـمَا, is the dual marker. Likewise, the third person singular masculine and feminine have the same dual form, that is هُمَا. After dropping the last letter of هُوَ and هِيَ, the dual suffix ـمَا is added, as in the table below.

ضَّمِيْر المُفْرَد ‘singular pronoun	ضَمِيْر المُثَنَّى ‘dual pronoun’
هُوَ	هُمَا
هِيَ
أَنْتَ	أَنْتُمَا
أَنْتِ
أَنَا	–

Unlike nouns, pronouns don’t get parsing marks. That is, they remain the same regardless of their position in the sentence. For this reason, the ن is not added to them; thus, there is no such thing as هُمَان and أَنْتُمَان.

Nominal Duals:

Singular nouns in Arabic are made dual by appending ـان to their ends. Therefore, singular nouns in the table above, مُهَنْدِس; لاعِب; طَبَّاخ; مُدَرِّسَة; and طَالِبَة, become مُهَنْدِسَان, لاعِبَان, طَبَّاخَان, مُدَرِّسَتَان, and طَالِبَتَان, respectively. Essentially, alif is the dual marker, while the ن replaces the parsing marks that occur with the singular, namely fatHah, DHammah, kasrah, or their corresponding tanween in the case of indefinite nouns.

The ا and ن change to ي and ن when the noun is in an object position, after a preposition, or the second noun of a genitive construction. Hence, the dual marker is ـان or ـين depending on the parsing mark of the noun, as in the following table:

Parsing Mark	مُفْرَد	مُثَنَّى
Nominative mark مَرْفُوْع	كِتَابٌ  ، الكِتَابُ سَيَّارَةٌ  ، السَّيَّارَةُ مُعَلِّمٌ  ، المُعَلِّمُ وَلَدٌ ، الوَلَدُ شَجَرَةٌ ، الشَّجَرَةُ	كِتَابَان ، الكِتَابَان سَيَّارَتَان ، السَّيَّارَتَان مُعَلِّمَان ، المُعَلِّمَان وَلَدَان ، الوَلَدَان شَجَرَتَان ، الشَّجَرَتَان
Accusative mark مَنْصُوْب	كِتَابًا ، الكِتَابَ سَيَّارَةً، السَّيَّارَةَ مُعَلِّمًا ، المُعَلِّمَ وَلَدًا ، الوَلَدَ شَجَرَةً ، الشَّجَرَةَ	كِتَابَيْن ، الكِتَابَيْن سَيَّارَتَيْن ، السَّيَّارَتَيْن مُعَلِمَيْن ، المُعَلِّمَيْن وَلَدَيْن ، الوَلَدَيْن شَجَرَتَيْن ، الشَّجَرَتَيْن
Genitive mark مَجْرُوْر	كَتَابٍ ، الكِتَابِ سَيَّارَةٍ ، السَّيَّارَةِ مُعَلِّمٍ ، المُعَلِّمِ وَلَدٍ ، الوَلَدِ شَجَرَةٍ ، الشَّجَرَةِ	كِتَابَيْن ، الكِتَابَيْن سَيَّارَتَيْن ، السَّيَّارَتَيْن مُعَلِمَيْن ، المُعَلِّمَيْن وَلَدَيْن ، الوَلَدَيْن شَجَرَتَيْن ، الشَّجَرَتَيْن

In the table above, the indefinite singular noun gets tanween (of fatHah, DHammah, or kasrah) based on the its parsing mark. The definite noun (the noun with ال) gets fatHah, DHammah, or kasrah. The dual noun gets ـان if it is nominative مَرْفُوْع and ـين if it is accusative مَنْصُوب or genitive مَجْرُور; therefore, the dual form with accusative and genitive mark is the same.

To further illustrate, let’s look at these sentences.

(1) الكِتَابَان فِي الشَّنْطَة.   ‘the two books are in the bag.’

(2) فِي الحَوْش سَيَّارَتَان. ‘there are two cars in the backyard.’

(3) نَامَ الوَلَدَان. ‘the two boys slept.’

(4) غَرَسْتُ شَجَرَتَيْن. ‘we planted two trees.’

(5) سَلَّمْنَا عَلَى المُعَلِّمَيْن. ‘we shook hands with the two teachers.’

(6) كِتَابُ الوَلَدَيْن جَدِيْدٌ. ‘the two boys’ book is new.’

In (1) and (2), the dual marker is ـان because الكِتَابَان and سَيَّارَتَان are nominative as they are in the position of مُبْتَدَأ (nominal) subject / topic.’ In (3), the dual marker is also ـان because الوَلَدَان is nominative as it is in the position of فَاعِل (verbal) subject. Both (nominal) and (verbal) subject always get the nominative mark. In (4) the dual marker is ـين because شَجَرَتَيْن is accusative since it is in an object مَفْعُوْل position. In (5) and (6), the dual mark is also ـيْن because المُعَلِّمَيْن and الوَلَدَيْن are genitive as the first is an object of a preposition اِسْم مَجْرُور, and the second is مُضَاف إِلَيْه, the second noun of a genitive construction.

Verbal Duals:

Like nouns, the dual marker with verbs is ـان; however, the ن is retained only with the indicative مَرْفُوْع form of the verb because it replaces the DHammah of the singular indicative form of the present verb. If the verb is subjunctive مَنْصُوب or jussive مَجْزُوم, the ن is dropped, as in these examples.

Indicative مَرْفُوع	Subjective مَنْصُوب	Jussive مَجْزُوم
الوَلَدَان يَأكُلان الجُبْن. ‘the two boys are eating the cheese.’	الوَلَدَان لَنْ يَأْكُلا الجُبْن. ‘the two boys will not eat the cheese.’	الوَلَدَان لَمْ يَأكُلا الجُبْن. ‘the two boys did not eat the cheese.’
البِنْتَان تَلْعَبَان فِي الحَدِيْقَة. ‘the two girls are playing in the garden.’	تُرِيْدُ البِنْتَان أَنْ تَلْعَبَا فِي الحَدِيقَة. ‘the two girls want to play in the garden.’	اِلْعَبَا فِي الحَدِيْقَة. ‘play in the garden!’
المُهَنْدِسَان يَخْرُجَان فِي الصَّبَاح. ‘the two engineers go out in the morning.’	خَرَجَ المُهَنْدِسَان كَي يَعْمَلا. ‘the two engineers went out in order to work.’	لا تَعْمَلا اليَوْم. ‘don’t work today!’

The dual marker ـان or ـا appears on the verb only if it is preceded by a dual noun. In the table above, the dual marker for indicative is ا and ن, while under the subjunctive and jussive the ن is dropped, the alif ـا, which is called the alif of the dual أَلِف المُثَنَّى, is the dual marker.

With past form of the verb, the dual marker is the alif, as in these examples:

الوَلَدَان أَكَلَا الجُبْن. ‘the two boy ate the cheese.’

البِنْتَان لَعِبَتَا فِي الحَدِيْقَة. ‘the two girls played in the garden.’

المُهَنْدِسَان خَرَجَا فِي الصَّبَاح. ‘the two engineers went out in the morning.’

Like the forms of the present, the alif shows on the past form of the verb only if it (the verb) is preceded by a dual noun. If the verb appears before the noun, it is not marked for dual, that it remains singular even if it followed by a dual noun, as in these examples:

أَكَلَ الوَلَدَان الجُبْن. ‘the two boys ate the cheese.’

لَعِبَت البِنْتَان فِي الحَدِيْقَة. ‘the two girls played in the garden.’

خَرَجَ المُهَنْدِسَان فِي الصَّبَاح. ‘the two engineers went out in the morning.’