IF Inside Of Where Clause

I have a sql statement that has several OR statements in it which work fine. It looks like bottom below.

What I need to know is can you put a IF statement in a where clause like this. Such as
WHERE convert(datetime, patient_.df_admit_date, 101) > = @tdate or
if patient_.dru = "yes" convert(datetime, patinet_.df_admit_date, 101) > = @tdate - 8 or

WORKIN STATEMENT
select
PATIENT.ACCOUNT_ID,patient_.DF_PPD_POS_NEG, PATIENT.LAST_NAME, PATIENT.FIRST_NAME, PATIENT.MIDDLE_INIT, PATIENT.OTHER_ID_NUMBER,
PATIENT_.DF_ADMIT_DATE, PATIENT_.DF_PPD, PATIENT_.DF_PPD_POS_NEG, PATIENT_.DF_PPDB_DATE,
PATIENT_.DF_XRAY_DATE, PATIENT_.df_ppd_read, FROM
{ oj development.dbo.PATIENT PATIENT INNER JOIN development.dbo.PATIENT_ PATIENT_ ON
PATIENT.COMPANY_ID = PATIENT_.COMPANY_ID AND
PATIENT.DEPARTMENT_ID = PATIENT_.DEPARTMENT_ID AND
PATIENT.ACCOUNT_ID = PATIENT_.ACCOUNT_ID}
where
convert(datetime, patient_.df_admit_date, 101) > = @tdate or
convert(datetime,patient_.df_ppd, 101) >= @tdate - 2 or continued!!!