should / ought to
应该，用来提建议或表达义务。
should do／ought to do
要点
- should 表示「应该」，用来提建议或表达义务。
- ought to 与 should 意思相近，语气稍强。
- 后面都接动词原形。
- should have done 表示「本应该做却没做」。
- should 也表"按安排预计会"：The train should arrive at nine.（火车该九点到）—— 这一层不是给谁提建议，说的是预期。
- be supposed to 常替 should 说话，且带"按规定、按说"：We are supposed to arrive before noon. —— 它的时态跟着 be 动词变（was / will be supposed to），should 却不能这么变。
- should 还藏在两个位置：条件句里表"万一"（If you should see him, tell him to call me.）；建议、要求后的从句里表"要"（I suggest that he go now.，英式常写 should go）。
例句
He should see a doctor.
他应该去看医生。
We should protect animals.
我们应该保护动物。
We should help each other.
我们应该互相帮助。
I should drink more water.
我应该多喝水。
The train should arrive at nine.
火车应该九点到。
We are supposed to arrive at the hotel before noon.
我们该在中午前到旅馆。
易错提示
- ought to 的否定是 ought not to，疑问把 ought 提前。
- should have done 是「本该做而没做」，带后悔语气。
- ought to 的 to 单独站着也要留着：He didn't go, but he ought to.（本该去）—— 省略到最后也不写 he ought。
- 提建议还有两条路：Why not + 动词原形（Why not ask her?）与 What / How about + -ing —— 三档里 should / ought to 是"该当义务"最重的那一档。