I defined tasks/events for my experiment but forgot to mark them while recording. Is there a way to mark them after recording?

Yes, you can use the Task Explorer window to mark tasks after recording. Seek to the time when you would like to mark the start of a task. Then click New in the Task Explorer window, choose the appropriate task, seek to the time when the task ends and click on the flag Set End on the right side of the new task created in the Task Explorer.