need 既可作情态动词也可作实义动词；should 表示"应该"，用于建议。
要点讲解
- need 作情态动词（多用于否定句和疑问句）：You needn't worry. 你不必担心；Need I go? 我需要去吗？
- need 作实义动词：need to do sth. 需要做某事：I need to buy some food. 我需要买些食物。
- needn't = don't have to 不必；must 的否定回答用 needn't。
- should 表示应该（建议、义务）：You should drink more water. 你应该多喝水。
- should 的否定 shouldn't 不应该：You shouldn't stay up late. 你不应该熬夜。
- should 也可表推测：It should be fine tomorrow. 明天应该天晴。
- 情态动词+have done：should have done 本应该做而没做（责备语气）。
经典例句
- You needn't come so early.
你不必来这么早。 - We should respect the old people.
我们应该尊敬老人。 - You shouldn't spend too much time on games.
你不应该在游戏上花太多时间。
易错提醒
- need 作实义动词时否定用 don't need to；作情态动词时否定用 needn't，两者不能混用。