trigger ClosedOpportunityTrigger on Opportunity (after insert, after update) {

    List<Task> taskList = new List<Task>();

    if(Trigger.isAfter){

        for(Opportunity opp : Trigger.new){
            if(opp.StageName=='Closed Won'){
                Task oppTask = new Task(WhatId=opp.Id, Subject='Follow Up Test Task');
                taskList.add(oppTask);
            }
        }
        insert taskList;

    } else {

        for(Opportunity opp : Trigger.old){
            if(opp.StageName=='Closed Won'){
                Task oppTask = new Task(WhatId=opp.Id, Subject='Follow Up Test Task');
            	taskList.add(oppTask);
            }
        }
        insert taskList;

    }
}