a. 表示现在( 指说话人说话时) 正在发生的事情。

　　We are waiting for you.

　　b. 习惯进行：表示长期的或重复性的动作，说话时动作未必正在进行。

　　Mr. Green is writing another novel.

　　(说话时并未在写，只处于写作的状态。)

　　She is learning piano under Mr. Smith.

　　c. 表示渐变的动词有：get, grow, become, turn, run, go, begin等。

　　The leaves are turning red.

　　It's getting warmer and warmer.

　　d. 与always, constantly, forever 等词连用，表示反复发生的动作或持续存在的状态，往往带有说话人的主观色彩。

　　You are always changing your mind.

　　典型例题

　　My dictionary ___, I have looked for it everywhere but still___ it.

　　A. has lost, don't find B. is missing, don't findC. has lost, haven't found D. is missing, haven't found.

　　答案D. 前句是一个仍在持续的状态，应用进行时，由于没有找到，其影响仍然存在，应用完成时，瞬间动词用于否定式时可用于完成时。