trigger PreventTaskCreation on Task (before insert, before update) {
    
    List<Task> relToCaseTasks = new List<Task>(); // store tasks that are related to cases
    Set<Id> caseIds = new Set<Id>(); // stroe the ids of the cases to which tasks are related to
    
    // appropriate error message
    String errMsg = 'Cannot create/update Task because case is already closed.'; 
    
    for(Task var : Trigger.new){
        if(var.WhatId != null){
            if(String.valueOf(var.whatId).startsWith('500')){ // check if the task is related to an case
                relToCaseTasks.add(var); // if yes then hold that task in a list
                caseIds.add(var.whatId); // also store the id of the case record
            }
        }
    }
    
    if(relToCaseTasks != null && relToCaseTasks.size() > 0){
        // query all the case records that had tasks assigned to them
        Map<Id, Case> idToCaseRecMap = new Map<Id, Case>([SELECT Id, Status FROM Case WHERE Id IN:caseIds]); // map to hold the case with their id as the key
        if(idToCaseRecMap != null && idToCaseRecMap.values().size() > 0){
            for(Task var : relToCaseTasks){ // iterate over the selected task records
                Case c = new Case();
                c = idToCaseRecMap.get(var.WhatId); // get the corresponding task record from the map
                if(c != null){
                    if(c.Status == 'Closed'){ 
                        var.addError(errMsg);
                    }                        
                }
            }
        }
    }
}