Complex sentence: The napkin is placed on the lap. Eating with ones mouth open, drinking while there's still food in the mouth, burping, or speaking while there's still food in the mouth is considered very rude.
Split into simpler sentences: The napkin is placed on the lap. Eating with ones mouth open, burping, smacking, and making other eating noises are considered uncivilized. Putting new food in the mouth, drinking or speaking while there's still food in the mouth is considered very rude.