With regards to the use of the singular at times and the plural at other times in Allaah’s addressing the people in the Qur-aan, Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah states:

And the address in the aayah is to the Prophet sall Allaahu ‘alaihi wa sallam but He said:

<<Wa qadaa rabbuka allaa ta’budoo illaa iyyaahu…>>

meaning:

<<And your (in the singular form) Lord has decreed that you (in the plural form) worship none except Him alone…>>
Soorah al Israa (17) aayah 23

And He did not say, that you (singular) worship none.

And the like of this in the Qur’aan is His statement, He the Most High:
<<O Prophet, when you (in the plural form) divorce the women…>>
Soorah at Talaaq (65) aayah 1

Therefore the first address is to the Messenger sall Allaahu ‘alaihi wa sallam and the second is general (to the people).  So what is the benefit behind this changing of the manner (of addressing)?

1) To draw attention, because getting the attention of the person being addressed is something desired by the one speaking. And this occurs here by changing the manner (of addressing).

2) That the Prophet sall Allaahu ‘alaihi wa sallam was the leader of his nation. And the address directed to him was directed to all of his nation.

3) An indication that whatever the Messenger sall Allaahu ‘alaihi wa sallam is addressed with, then it is for him and for his nation – except for something where there is a proof that it is specifically for him.

4) And in this aayah in particular, there is an sign that the Prophet sall Allaahu ‘alaihi wa sallam was one who was under the control of the Lord, and not (himself) a Lord and Nurturer – a servant, and not one who is to be worshipped.  So he comes under His statement:

<<…that you (in the plural form) worship..>>

And sufficient for him in nobility is that he be a servant of Allaah, the Mighty and Majestic.

(Al qawl ul mufeed (1/32 to 33) of Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah. Translated by Nasser ibn Najam)