1．Be careful！Don't throw your cigarette out of the window．小心！不要把你的烟头扔到车窗外。（＝Be careful not to throw your cigarette out of the window．）
Be careful．．．常用来警告对方。又如：
Be careful of rats ／flies ／snakes ／．．．！当心老鼠／苍蝇／蛇／……！
Be careful with your handwriting ／pronunciation．．．！注意你的书写／语音……！
Be careful not to stand close to the wire！小心不要靠近电线！
Don't do sth．也是一种告诫。例如：
Don't play in the street！不要在街上玩耍！
Please don't throw paper on the floor！请不要朝地板上扔纸！
2．Look out！There's a kangaroo！当心！有一只袋鼠！
Look out！也是一种警告句式，提醒对方要当心，要小心留神。例如：
Look out！There is danger ahead．当心！前面有危险。
Look out for the snags！当心触礁！
3．Take care．The rocks are wet．小心！岩石潮湿。
take care和look out意思相近，也是用来告诫对方要小心、当心的一种说法。例如：
Take care．The path is slippery．当心。小路很滑。
Take care not to make careless mistakes in your examination pa- per．当心不要在试卷上出粗心错误。
4．If you do that，you may start a fire．如果你那样做会引起火灾的。
Ifyou．．．，youwill／may／might．．．是一种禁止、劝诫句式。这种
表达意思与“Don't do that，you may．．．”相近，但语气要委婉得多。
例如：
If you tie it to that old branch，it'll break．如果你把它系在枯树枝上，枯树枝会断的。
5．But you mustn't smoke while you're walking around in the bush either．但是你在丛林中行走时也不许抽烟。
Youmustn't／can't．．．是禁止句式。这种句式意同“You are not
allowed ／permitted to．．．”。例如：
You mustn't make noise in the reading room．你不许在阅览室喧哗。
