How to use case condition in sql server

select AccountDID,Productslice,Productcount,LCSCount from (select coalesce (a.AccountDID,b.AccountDID) as AccountDID,a.ProductSlice,

sum(isnull(a.ProductCount,0)) ProductCount,sum(isnull(b.LCSCount,0)) LCSCount,b.RdbSliceCode from #ProductAudit a

full outer join #LCSAudit b on a.AccountDID=b.AccountDID and a.ProductSlice=b.RdbSliceCode

group by a.AccountDID,b.AccountDID,a.ProductSlice,b.RdbSliceCode)a where LCSCount<>ProductCount

here #ProductAudit table contain one column i.e levelCapability and #LCSAudit contain one column i.e usergroupname

So i want to use case in above query like..the condition is..

if in #ProductAudit table ...levelCapability =="recruitment" and in #LCSAudit table ..... usergroupname==null than i want to select the account did..

How can i do it?