trigger MultipleRealScenariostrigger on Opportunity (before insert, before update, before delete, after update ){
         //varaibles hold open and closed types StageName
         Set<string> openstages = new Set<string>();
         Set<string> closedstages = new Set<string>();
        
        //Add open stages in the set
        openstages.add('Prospecting');
        openstages.add('Qualification');
        openstages.add('Needs Analysis');
        openstages.add('Value Proposition');
        openstages.add('Id. Decision Makers');
        openstages.add('Perception Analysis');
        openstages.add('Proposal/Price Quote');
        openstages.add('Negotiation/Review');
        //Add close stages in the set
        closedstages.add('Closed Won');
        closedstages.add('Closed Lost');
        
        //Get list of opp with stagename as Prospecting and userinfo
        List<Opportunity> opplist = [Select id from opportunity where StageName =:'Prospecting' and StageName =:'Closed Lost' Limit 2];
        String userprofile = [Select name from profile where Id = :UserInfo.getProfileId()].name;
        List<Task> lsttask = new List<Task>();//List of Tasks
        Set<ID> setOppIds = new Set<ID>();//Holds set of opportunity Ids
        
        //Iterate through opportunity object
        for(opportunity o : Trigger.new){
            //trigger of contextvaraibles isinsert and isupdate
            if(trigger.isinsert||trigger.isupdate){
                if(trigger.isupdate){
                    //Condition to check stagename with old value if it is close can not be changed to open stage
                    if(Trigger.oldMap.get(o.Id).StageName == 'Closed' && o.StageName == 'Open'){
                        o.addError('Stage can not be move from closed to open');
                        return;
                    }
                    /*if(closedstages.contains(system.Trigger.oldMap.get(o.Id).StageName)&& openstages.contains.(o.StageName)){
                        o.adderror('Stage can not be moved back form Close to Open');
                    }*/
                }
                //Verify condition that opportunity stagename is closedLost
                if(o.StageName == 'Closed Lost' && o.StageName != Trigger.OldMap.get(o.Id).StageName){
                    setOppIds.add(o.Id);//Add opportunity ids in variable setOppIds
                }
                //Check the opplist variable size if size is greater then 1 through error
                if(opplist.size()>1){
                    o.adderror('Already one Opportunity with open status present');
                }
            
            //Trigger for context variable isupdate and isdelete    
            if(trigger.isupdate||trigger.isdelete){
                //check condition that stagename as closedlost and userprofile is sys admin
                if(o.StageName =='Closed Lost'&& userprofile !='System Administrator'){
                //Through error while user not equal to sys admin trys to delete or modify closed lost opportunity
                    o.adderror('Only Admin can delete once opportunity status is closed');
                }
            }
           }
        }  
        //if opportunity stage name is ClosedLost then associated tasks status is updated to Completed
        If(setOppIds.size()>0){
            //Fetch list of task associated with opportunity stored in setOppIds varaible
            lsttask = [Select id, Status from Task where WhatId in :setOppIds];
                if(lsttask.size()>0){
                    for(Task tsk : lsttask){
                        tsk.Status = 'Completed';
                    }
                    update lsttask; 
                }
        }
    }

 Full trigger code