要说某件事是个好主意，用 should + 动词原形。不管是 I、you 还是 he，形式都一样，后面不加 to。
should + 动词原形
You should study for the test.
She should see a doctor.
You should try Farville Restaurant.
Not "You should to study more." — say "You should study more.". should 后面不加 to：直接 should + 动词原形。
试一试: She ___ a doctor about that cough. (should see / should to see / should sees) 答案: should see — should 后面用动词原形，不加 to，也不加 -s：should see。"Should to see" 多加了一个 to，"should sees" 多加了一个 should 本来就不该有的 -s。
shouldn't + 动词原形
要说某件事不是好主意，用 shouldn't + 动词原形。（shouldn't = should not。）
You shouldn't eat so much sugar.
He shouldn't drive so fast.
We shouldn't be late.
Not "You don't should eat so much sugar." — say "You shouldn't eat so much sugar.". 否定形式是 shouldn't，绝不是 "don't should"。
试一试: He ___ so fast on this road. (shouldn't drive / don't should drive / shouldn't drives) 答案: shouldn't drive — 否定形式是 shouldn't，不是 "don't should"。后面的动词还是原形：shouldn't drive（不是 "drives"）。
Should I ...?
要问对方的建议，用 Should + 这个人 + 动词原形开头。
Should I call him?
Should we go now?
What should I do?
试一试: ___ I call Amina now, or is it too late? (Should / Do / Am) 答案: Should — 问建议要用 Should 开头：Should I call her? "Do I call" 问的是习惯性的事，"Am I call" 不是正确的说法。
试一试: You look tired. You ___ rest, and you ___ stay up so late. (should … shouldn't / shouldn't … should / should … should) 答案: should … shouldn't — 休息是好主意 → should rest。熬夜是坏主意 → shouldn't stay。好主意用 should，该避免的事用 shouldn't。