这得看划线部分
see a dentist 应为 what should you do when you have a toothache
when you have a toothache 应为 when should you see a dentistyou should better take a good rest if you have a cold.变否定句呢？You should betternot .......you should（ ） better take a good rest if you have a cold这个句子很少见，一般用had better那只能用shouldn't .you had（ ） better take a good rest if you have a cold.变否定句