Why in the quran Allah use the you ( singular) and sometimes you

Singular form almost always refers to the Prophet Muhammad (SAW). For example:

And who believe in what has been revealed to you, [O Muhammad], and what was revealed before you, and of the Hereafter they are certain [in faith]. (2:4)

Sometimes it may be singular outwardly referring to the Prophet Muhammad (SAW), but refer to Muslims or people in general by extension through the Prophet. It is simply a way of speaking. For example:

The truth is from your Lord, so do not be among the doubters. (3:60)

The message is for people in general (since the Prophet does not doubt) but it is said in proxy through the Prophet (SAW).

Plural form can refer to different groups of people including but not limited to Muslims, Bani Israel, and all humans in general depending on context.

Muslims: "Neither those who disbelieve from the People of the Scripture nor the polytheists wish that any good should be sent down to you from your Lord. But Allah selects for His mercy whom He wills, and Allah is the possessor of great bounty." (2:105)

Bani Israel: "O Children of Israel, remember My favor which I have bestowed upon you and fulfill My covenant [upon you] that I will fulfill your covenant [from Me], and be afraid of [only] Me." (2:40)

Mankind: "O mankind, worship your Lord, who created you and those before you, that you may become righteous" (2:21)