1. see sb. do sth “看见某人做了某事” 强调动作的全过程,常与every day; often等连用.

　　see sb. doing sth. “看见某人正在做某事” 强调动作正在进行.

　　如: I saw her go across the street. 我看见她过了马路

　　I saw her going across the street. 我看见她正在过马路.

　　[类似的有watch,hear,feel 等这类感观动词.]

　　2. join sb. 表示 “加入某人的行列” “和某人在一起”

　　join + 组织 表示 “加入某个组织”

　　take part in 表示 “参加/出席某个活动”

　　如: Will you join us?

　　I will join the skiing club.

　　She is planning to take part in the high jump.

　　3. leave… 离开……

　　leave for… 动身去…/离开到…

　　如: They are leaving Beijing tomorrow. 明天他们要离开北京.

　　They are leaving for Japan the day after tomorrow. 后天他们要前往日本.

　　4. how long 表示“多久(时间)”; 提问时间段.

　　how often 表示 “多常; 多久一次”; 提问时间的频率.

　　如: They will stay in Beijing for a week. → How long will they stay in Beijing?

　　He plays basketball twice a week. → How often does he play basketball?

　　5.be good at (doing) sth. = do well in (doing) sth. 擅长于(做)某事

　　如: She is good at (playing) baseball. = She does well in (playing) baseball.

　　6.make sth/sb + adj. 使某物(某人)在某种状态

　　keep …sth/sb + adj. 保持某物(某人)在某种状态

　　如: Playing soccer can make your body strong.

　　Swimming can help to keep your heart and lungs healthy.