I have following trigger which creates a log in  the custom object whenever a task is modified.

There are two problems the trigger is creating a log record even when the new task is created. It should only created the record when the task is modified.

Also it is creating a log for all the objects. I only want the log to be captured for Accounts/Contacts i.e whatid should only be account/contact record.

Can someone please help in refining the trigger.

Thanks in advance.

trigger CommentLog on Task (before update) {
    List<CommentLog__c> newcomments = new List<CommentLog__c> ();
    Map<Id, Task> mTask = new Map<Id, Task> ();
    
    //build the task map first
    for(Task t: [select Id, owner.name,LastModifiedBy.name,what.name from Task where Id in : Trigger.old]) {
        mTask.put(t.id,t);
    }
    
    //build comment log
    for(Task t: trigger.old) {
      CommentLog__c c = new CommentLog__c();
      Task tt = mTask.get(t.Id);
      c.Previous_Comment__c=t.Description;
      c.CommentDate__c=t.LastModifiedDate;
      c.Name= (tt!=null?tt.LastModifiedBy.name:null);
      c.Task_owner__c = (tt!=null? tt.owner.name:null);
      c.type__c='Task';
      c.Taskid__c=t.id;
      c.Link_to_Task__c=URL.getSalesforceBaseUrl().toExternalForm() + '/' + t.id;
      c.whatid__c=URL.getSalesforceBaseUrl().toExternalForm() + '/' +t.whatid;
      c.Account_Contact_Name__c=(tt!=null?tt.what.name:null);
      c.Account_Contact_ID__c = t.whatid;
      newcomments.add(c);
    }
    //insert comment log list
    insert newcomments;
}