Why does Allāh sometimes use the ʿArabic plural pronoun while referring to Himself while in other instances, He makes use of the singular pronoun?
In replying to this question, we first put forth the two below preambles:
One: The scholars of theology (ʿāqāīd) – also known is ʿilm al-kalām – have presented various logical proofs to establish the oneness of Allāh . The meaning of Allāh being one is that it is only He who is independent and needless of anyone or anything in existence — He creates; He sustains; He guides; He causes to die, etc… In the science of theology, these things are referred to as the Oneness (tawḥīd) in creation; the Oneness (tawḥīd) in sustenance; the Oneness (tawḥīd) in ontological guidance, etc…
Two: What has been mentioned above does not mean that someone or something – in the field of creation, sustenance, guidance and other such things – has no role to play in these areas of life. Rather, the wisdom of Allāh dictates that the acts of creating, sustaining, guiding — and rather — most of the actions which are carried out are done through [natural] means.
For example, even though Allāh is directly able to — and without any other external means — cure the sick, feed the hungry or guide those who are misled, however based on its own wisdom, it is much more admirable that each and every one of these actions is carried out through their specific [natural] means and that every action is thus carried out [in the world through the customary sources]. Therefore, the hungry people are satiated with the bread which they themselves [humanity] have prepared; the sick are healed through the medicine which they themselves have manufactured and which has been administered by a doctor who himself has been trained in these regards, and so on. Of course, the bread and the doctor and the medicine and all of these are only effective by His permission and all of them have their own specific outcome [within them] and therefore, when the medicine and the doctor; or the bread and the water and all of these things take their continuous existence from Him and through His permission, produce the desired effects [which they are made for], then such a cause and effect relationship which exists does not go against the belief in tawḥīd and the oneness of Allāh in the various aspects. Such a thing is also never considered as being partners with Allāh – rather, they are all creations of Allāh and in all aspects of their existence, including the effect which they carry, are all reliant upon Him.
Therefore, by keeping what we have just stated in mind, let us now return to the initial question and offer our reply to it. We state that: Allāh is only One and anytime He informs of His actions — as He is One — it stands to reason that [when He speaks of His acts] they should always be mentioned with singular words and also singular pronouns. Therefore, we see that many times in the Qur’ān such phrases have been used.
However we see that in ʿArabic and also sometimes in other languages, the speaker many want to, for numerous reasons, say “WE did this thing” rather than saying “I did that thing” – but why?
Below we present some of the proofs for why this is done:
A. The Noble Qur’ān has chosen to make use of the conversational language which the people use as the best way to communicate with them. Thus, sometimes it is seen that in daily conversations, people may employ statements such as, “That person is manly [possesses the trait of chivalry]” whereas it is possible that they are referring to a female and not a male [but are still using the phrase manly in its general sense]. In the Noble Qur’ān we see the phrase which reads, (O mankind! You are the poor in your relation to Allah.) In this verse, the pronoun used in the word “you” (أنتم) is the masculine, however it is a general address being made to both women and men, and anyone with intellect would understand that the meaning of this pronoun is all of humanity and not only men. In regards to the plural and singular pronouns we also see that many times people will, in place of the singular pronoun, make use of the plural pronoun in their speech. For example people sometimes say, “We [all] came however all of you were not home.” In this sentence, the pronoun in ‘we’ and the pronoun in ‘you all’, even though they are plural, however the meaning of the speaker and what is understood by the listener is the singular pronoun. In addition, in order to show honour, esteem and respect and to confer a great status on an individual, the plural pronoun is used. In regards to Allāh, the Noble and Grand, we also see that for greatness and [His] eminence, Allāh uses the pronoun for ‘we’ or ‘نحن’.
In any case, in order for a speaker to remind the listener of the speaker’s own greatness, rather than saying “me”, he will make use of the word “we” which will denote on the greatness and eminence of the narrator.
In an example in the Qur’ān we see that in the first verse of Sūrah al-Fatḥ (Chapter 49), that Allāh says:
“Indeed, We have given you [O Muḥammad] a manifest victory.”
Some commentators of the Qur’ān have stated that in this verse as Allāh wishes to show His own greatness, He has stated that “WE have given you the conquest [of the city of Mecca].” However as to why it is seen that in this verse, the phrase of “WE” has been used, it is due to the fact that this method of communication which points to [His] greatness is much more suitable with the mentioning of the ‘victory’ and this same point can also be applied to the verse of the Qur’ān which reads:
“Indeed We have sent you [O Muḥammad] as a witness…”
B. Sometimes, in order to show the greatness of the action which has been performed, rather than saying, “I had done such and such act”, the individual speaking would actually say, “We did such and such act” – for example:
“Indeed We have revealed it [the Noble Qur’ān] on the Night of Grandeur” or “Indeed We have granted you [O Muḥammad] al-kawthar.”
And such examples actually go back to the greatness of the Qur’ān and the [spring] of al-Kawthar.
C. Sometimes, in order for Allāh to draw our attention to the fact that there are causes and effects which He has put into place for the performance of certain actions, He would, in place of saying, “I have done such and such a thing”, would actually use the word “We” and therefore, in the fourth verse of Sūrah al-Raʿd (chapter 14), we read the following commentary mentioned in al-Mizān fī Tafsīr al-Qur’ān: “And this same phraseology of ‘We’ – (We make some of them excel over others) for the first person plural is not free from giving the feeling that there are divine causes besides Allāh that work according to His command that end at Him, the Glorified.”
We also see the same opinion under the commentary taken from Tafsīr-e Namūna under verse 61 of Sūrah Yūnus (chapter 10) in which it is stated: “The usage of the plural pronoun when speaking about Allāh despite the fact that His pure essence is only ONE from all possible aspects is done to show His lofty status and also so that we understand that He is always controlling those who are under His command to fulfill His orders and in the obedience to His commandments are ever ready and waiting [to comply] and in reality, the discussion does not only relate to Him, but rather it is about Him and also all of those who are obligated to obey Him.”
It is important to note that it is possible that in one verse of the Qur’ān, we may actually see all three explanations given above combined – meaning that the pronoun to speak about Allāh may come in the plural to not only show the greatness of the one who is performing the action, but also to show the greatness and eminence of the action itself which is being done and also to draw the reader’s attention to the cause and effect relationship – and this can be seen in the verse of the Qur’ān below in which all three examples are present:
“Indeed We have revealed it [the Noble Qur’ān] on the Night of Grandeur”
 There is a ḥadīth in al-Kāfī, vol. 1, pg. 183, in the section of knowing the Imām and referring things back to him, which states:
“Allāh disdains that things are carried out [in the world] except through their [natural] means.”
 If this is the case, then the question which is raised is that in certain instances, why does Allāh use the singular pronoun has no basis on our discussion, and the only question which remains to be answered is why does Allāh not always make use of the singular pronoun? This is what we will answer in the next portion of this discussion.
 For example in Sūrah al-Ghāfir (30), verse 60 [and other places] we read:
“Call upon Me, I will answer you.”
 Al-Qur’ān, Sūrah al-Fāṭir (35), verse 15
 For more details, refer to the book Questions and Answers by Shaykh Nāsir Makārim Shīrāzī, vol. 3, pg. 256
 Al-Mizān fī Tafsīr al-Qur’ān, Ṭabā’ṭabā’ī, Sayyid Muḥammad Ḥusayn, vol. 18, pg. 385
 Ṭabā’ṭabā’ī, Sayyid Muḥammad Ḥusayn, Al-Mizān fī Tafsīr al-Qur’ān, vol. 11, pg. 401
 Shīrāzī, Nāṣir Makārim, Tafsīr-e Namūna, vol. 7, pg. 358; Maʿārif Qur’ān, Miṣbāḥ Yazdī, pg. 106-114