well, i don't think i've ever seen the IsDate() function used that way. the
function "Returns a Boolean value indicating whether an expression can be
converted to a date." (per Access Help.) unless your fields are *not*
date/time data type, you don't need to test whether the values are valid
dates. i think what you're trying to do is see whether the fields contain a
date value or whether they're *null*. i suppose the IsDate() function might
give the same result, but having never used it in that manner, i don't know
if there are circumstances where it might return unexpected results. suggest
you try instead

=IIf([Fo_Date] Is Not Null, "Delivered", IIf([T_Date] Is Not Null,
"Ordered", IIf([S_Date] Is Not Null, "Approved", "Waiting Approval")))

if you really want to use the IsDate() function for some reason, you may
need to test for a boolean value, as

=IIf(IsDate([Fo_Date])=True, "Delivered", IIf(IsDate([T_Date])=True,
"Ordered", IIf(IsDate([S_Date])=True, "Approved", "Waiting Approval")))

hth