null or ""

Null means no data exists for a given Field in a Table.
The data is unknown.

"" is a Empty Length String that has not been initialized.

Dim X as String
at this point. X="" ... which IS valid data.

X="Duggan"
Now it's initialized

I tried to do the coding.  
If PDState = "Y" then
   PDUser = ""
end if

when I ran the query if PDUser is not null, one of the case shows in the query, but the PDUser is empty, so I changed to
If PDState = "Y" then
   PDUser = Null
end if
when I ran the same query, the query ran correctly.

that's why I wonder if I should use Null for datefield and text field or I shohld use "" for datefield and Null for text field.

I AVOID Empty Length Strings at all costs as Table Field values  because visually ... you cannot tell a Null from a ELS.

You can use this test

IF Nz(x,"")=""  Then  ' whatever

or

IF Nx(x,"") > "" Then ' whatever

"if I should use Null for datefield and text field or I shohld use "" for datefield and Null for text field."

You should use Null for Both ... then you only have to worry about testing like this .... IsNull (SomeField)

so I should never use "" in coding or I can use "" for if the field is a number field (like 12345)?

Thank you!!  I wish I can give you all 500 points each.