Should используется для дачи советов или рекомендаций.
Shouldn't используется для предупреждения о нежелательных действиях.
Doctor Brown is a famous expert in healthy living. He says that you should eat more vegetables to stay fit. However, he warns that you shouldn't eat too much sugar every day. To have a strong heart, you should exercise regularly.
Ответ: should, shouldn't, should