宾语从句：it might have been prevented had he received the dental care he needed （用了虚拟语气,对过去的假设）
状语从句：had he received the dental care he needed （省略了if ,把助动词had提前）
定语从句：he needed （省略了关系词that或者which）
整句话的意思是：大家说如果他接受所需的牙齿治疗,就能避免（这种病）了.
宾语从句：it might have been prevented had he received the dental care he needed （用了虚拟语气,对过去的假设）
状语从句：had he received the dental care he needed （省略了if ,把助动词had提前）
定语从句：he needed （省略了关系词that或者which）
整句话的意思是：大家说如果他接受所需的牙齿治疗,就能避免（这种病）了.