Be doing sth when..一系列“正在做.忽然做."哪个加动词原形?哪个通常用在过去完成时?为什么Be doing sth when..be about to do sth when..be on the point of doing sth when..had done sth when

be doing sth.when 如何对be doing sth when造句 be on the point of doing sth.when...be on the point of doing sth.when...是固定句型, be always doing sth . be doing sth. 用 Sb who do sth be like sb else doing sth else who,when/once.造句 be worried doing sth 还是be worried about doing sth when+doing 还是when+to do sth. be worthy of sth / doing sth. be worth sth./doing sth.造句, there be sb/sth doing be afraid of doing sth be busy doing sth 例句 be interested in doing sth be busy in doing sth be long in doing sth be behind in doing sth be stop from doing sth