A child with learning differences, like ADD or dyslexia, may try to use up time to distract you. This is effective, even when you get angry and frustrated because you, too, will use up lesson time.
Pull back and think analytically. Resist the urge to ask, "Why is Amy wasting my time?"
Instead, ask "Why is Amy using her time this way? What is she getting, or getting out of, by talking right now?"
When a student suddenly starts talking during a lesson figure out WHY before you listen to the content. There are many possibilities. Sometimes they're just chatting. This can range from lovely to irritating depending on your patience level and frankly, how much you are interested in what they're saying. But when there's less "teaching" going on, sometimes the student is teaching you something more important than what you're attempting to teach them. Listen for what's below the surface.