Why does Quran use 'We' and 'I' both words for Allah.

1. Quran gives the answer in the Quran itself by declaring Allah One and Only again and again. How can a book which has been written exclusively to deal and address the issue of one God, declare Him two through its grammar. No, it's not possible. If it's grammar or pronouns indicate of him being two, it's simply means that the there has been a mistake  in understanding the different linguistical usage of pronouns.

2. No Idolaters or Mecca Mushrik, who used to worship many gods at that time had ever objected that on one hand Quran declares there is only one God and on the other hand it speaks that there are many gods. If it was so as questioned, it would have been in favour of Mushrik to prove that Quran itself confirms the existence of multiple gods as they believed but they didn't do so because being Arab, they very well knew the Arabic linguistical usage of we, our, us etc. against singular pronoun depending upon the context. 

3. Similarly, for last fourteen hundred years, no follower of Islam had ever taken the meaning of multiple gods from this usage of plural pronoun which shows that they know that in Arabic language, it's a linguistical style to use We instead of I to show respect and dignity. No Arabic linguistic scholar has objected this due to the same reason.

4. In poetic literature, arrangements of words and their collective meaning in a sentence are more important than the order of the words or their individual meanings. Similarly, in poems, the denotation of words and their context are more important than the nouns and pronouns used in it.

Being a poetic masterpiece, whenever Quran refers Allah in the second and third person, it uses singular pronoun and when it refers Him in the first person, it uses singular and plural pronouns both. Nauzubillah, if Allah were two , the Quran would have used plural pronoun for Allah in the second and the third person as well.

In many languages, plural pronouns are used for first person but never for second or third person. When 'we' is used in place of 'I',  it is called “royal we” and many languages including Arabic, English, Hebrew and Persian provide this usage. Even in Hindi and Urdu, we say हम instead of मैं while showing our mannerism.

In the Quran, the singular form has generally been used, when Allah speaks about His love, care and closeness and forgiveness for His servants and the plural form has been used when Allah speaks about His power, majesty, glory, great deeds or when He speaks about His anger and wrath for the sinners and criminals. There are some exception to it also as its natural.

5. Plural pronouns have also been used in other religious scripture such as in Bible at many places.

"Then God said, Let Us make man in Our image", (Bible: Genesis: 1:26)

Thus it is evident that the usage of plural form of pronouns for Allah is just a linguistic and poetic style.