All Praise and Thanks be to Allaah, the Cherisher and Sustainer of the Worlds.
It is a feature of literary style in Arabic that a person may refer to himself by the pronoun ‘nahnu’ (we) for respect or glorification. He may also use the word ‘ana’ (I), indicating one person, or the third person ‘huwa’ (he). All three styles are used in the Qur’an, where Allaah addresses the Arabs in their own tongue.1
“Allaah, may He be glorified and exalted, sometimes refers to Himself in the singular, by name or by use of a pronoun, and sometimes by use of the plural, as in the phrase (which translated means): ‘Verily, We have given you a manifest victory” [al-Fath 48:1], and other similar phrases. But Allaah never refers to Himself by use of the dual, because the plural refers to the respect that He deserves, and may refer to His names and attributes, whereas the dual refers to a specific number (and nothing else), and He is far above that.”.2
“These words, innaa (“Verily We”) and nahnu (“We”), and other forms of the plural, may be used by one person speaking on behalf of a group, or they may be used by one person for purposes of respect or glorification, as is done by some monarchs when they issue statements or decrees in which they say “We have decided…” etc. [This is known in English as “The Royal We” – Translator]. In such cases, only one person is speaking but the plural is used for respect.
The One Who is more deserving of respect than any other is Allaah, may He be glorified and exalted, so when He says in the Qur’an innaa (“Verily We”) and nahnu (“We”), it is for respect and glorification, not to indicate plurality of numbers. If an aayah of this type is causing confusion, it is essential to refer to the clear, unambiguous aayaat for clarification, and if a Christian, for example, insists on taking ayaat such as “Verily, We: it is We Who have sent down the Dhikr (i.e., the Qur’an)” [al-Hijr 15:9 – translation of the meaning] as proof of divine plurality, we may refute this claim by quoting such clear and unambiguous aayaat as “And your god is One God, there is none who has the right to be worshipped but He, the Most Beneficent, the Most Merciful” [al-Baqarah 2:163] and “Say: He is Allaah, the One” [al-Ikhlaas 112:1] – and other aayaat which can only be interpreted in one way. Thus confusion will be dispelled for the one who is seeking the truth. Every time Allaah uses the plural to refer to Himself, it is based on the respect and honour that He deserves, and on the great number of His names and attributes, and on the great number of His troops and angels.” 3
“In regards to the presence of the "us" and "our" in the Old Testament, this is actually a proof against Christian belief since they were never understood in the way Christians understand them until the time of ‘St’ Paul or later. In Semitic usage, it is customary for one in authority to speak of himself in the plural. For examples of this in the Bible, see 2 Samuel 16:20 and Ezra 4:16-19. Almighty God, in the Qur'an, uses the Arabic word for "We" when He makes some statements, but no one who knows the Arabic language ever understood it as an argument for indication of plurality. This is what we call the "Plural of Majesty", and it is mentioned in almost any book on Arabic grammar.
English has much the same thing. When a King or Queen issues a proclamation, they say "We, the King of England..." This is also done by monarchs in Arabic-speaking countries. Who are they talking to when they do this? Their intended audience of course, not themselves. The examples given above (i.e. 2 Samuel 16:20 and Ezra 4:16-19), also demonstrate this.
So it is suffice to say that plural forms, especially in Arabic and Hebrew and even English, don't necessarily imply plurality. The claim that "plurality of the Godhead" is implied in the language of the Old Testament (which is admittedly by the way imprecise) is just an invention by Christians in order to fill a previously determined theological need. The Greek translation of the Old Testament (known as the "Septuagint" translates "'Eloyhim" the plural form of God, as "ho Theos" (i.e. the God). This is a SINGULAR translation which was done at least two centuries prior to the coming of Jesus, peace be upon him, and shows that the implication of plurality in these words is a purely Christian invention. In spite of all this, if Christians still want to believe that the "Godhead", as they often refer to it as, is "plural", then that belief is to their own discredit.
It should also be mentioned that it's not just the incoherence of Christianity that leads one to these conclusions, because on top of all of this we have a new, CLEAR and unambiguous revelation - the Qur'an - that contradicts the Christian claims. Whether one agrees that the Qur'an is Almighty God's final revelation to mankind or not, the guidance in it is CLEAR. That the Qur'an's central message is the Unity of God is indisputable.
The message of the Bible isn't so clear. If God is merciful and really wants all men to be saved by believing in the Christian doctrines and beliefs, why didn't Jesus make the message clear? Why is most of the New Testament written by Paul, who still didn't make things clear?
Almighty God, as He describes Himself in the Qur'an, is Merciful and Compassionate towards His Creation, and one way which He manifests this is by making His revelations clear and comprehensible - even to the limited human mind.” 4
And Allaah knows best.
[1 ]Fataawa al-Lajnah al-Daa’imah, 4/143  Al-‘Aqeedah al-Tadmuriyyah by Shaykh al-Islam Ibn Taymiyah, p. 75 ibid, p. 109 Abu Iman 'Abd ar-Rahman Robert Squires (www.Muslim-Answers.org)