①Will, still watching, saw the cat behave curiously.
②When it had gone past he crossed the road, keeping his eyes on the spot where the cat had been investigating.
③While sitting by a pool, Ferdinand sees a frog.
④Before leaving his kingdom, he calls his son to him.
⑤Not knowing her address, I can't write to her to express my gratitude.
⑥Having lived with the girl for 5 years, we all know her very well.
[语法领悟]
(1)六个例句中的动词ing形式的逻辑主语都是句子的主语。
(2)由例句⑤可知，动词ing形式的否定式是在ing形式前面加not。
(3)在例句③中，动词ing形式短语前用while，为了强调与谓语动词的动作同时发生；在例句④中，动词ing形式短语前用before，为了强调在谓语动词的动作之前发生。
(4)由例句⑥可知，动词ing形式的完成式为having_done。
