if (Trigger.isBefore && Trigger.isDelete) {
    for (record r : trigger.Old) {
        if (r.ClosedDate > system.today()) {
            r.addError('Cannot delete records that have not yet closed');
        }
    }
}

trigger validation_using_Trigger on Account (before delete) {
 for(Account acc:trigger.old){
    if(acc.AnnualRevenue < 2000){
       acc.adderror('Annual revenue cannot be less than 2000');
    }
 }
}