I need Nested IF Function in excel cell for the below query

I have four columns (Age,Freq, Day,Month) and the condition is as follows

If Age is 0-30 and Freq is Monthly and Day is <61 and Month is current or
previous month then the result should be "gradeA"

If Age is 0-30 and Freq is Monthly and Day is <61 and Month is Not current
or previous month then the result should be "gradeB"

If Age is 1-30 and Freq is Monthly and Day is 61 and Month is current or
previous month then the result should be "gradeF"

If Age is 1-30 and Freq is Monthly and Day is 61 and Month is Not current
or previous month then the result should be "gradeD"

If Age is 31-60 and Freq is Queratly and Day is <121 and Month is current
months(example:from current month take three months backward oct,sep,Aug are
considered as current month) then the result should be "gradeH"

If Age is 31-60 and Freq is Queratly and Day is 121 and Month is Not
current months then the result should be "gradeK"

Queartly always considered as three months from current month
Annually always considered as 12 months from current month

If Age is 61-90 and Freq is Annually and Day is <180 and Month is current
months then the result should be "grade Annual"

ELSE : FALSE