预习目标： 1. learn what members of the sentence the present participle can act as; 2. learn to identify the understood subject of the present participle; 3. learn when to use “having done”. 预习任务： a. read the points on page 28, and then fill in the blanks: 现在分词在句子中可以充当：a. _______, b. _______, c. _______, d. _______ do part a and b on p. 29. b. 观察下列各组句子，可知：当现在分词在句子中充当_________和_________的时候，其“逻辑主语”（understood subject，即分词动作的执行者或承受者）就是句子的主语；现在分词作定语的时候，其“逻辑主语”是___________________；现在分词作宾语补足
