将来完成时是一种英语时态，表示在将来某一时间之前已经完成的动作。

定义上，将来完成时描述的是一种在将来的某个时间之前就已经结束的动作或事件。例如： "By the time we arrive, she will have prepared dinner."

结构上，将来完成时的肯定句形式是主语 + will + have + 过去分词。例如："By eight o'clock tomorrow, she will have finished her homework."

否定句的构成是主语 + will not (won't) + have + 过去分词。例如："By eight o'clock tomorrow, she will not have finished her homework."

疑问句的形成是将助动词 will 提前到主语前。例如："By eight o'clock tomorrow, will she have finished her homework?"

特殊疑问句则是将特殊疑问词放在句首，后面加上 will 和主语。例如："By eight o'clock tomorrow, when will she have finished her homework?"

将来完成时的基本用法包括表示在未来某个时间之前结束的动作或事件，例如："By the time the party starts, I will have arrived."

它也可以用于表示推测，例如："By the time the movie starts, I don't think he will have arrived."

时间关键词有 before, by the time, by + 特定时间, when, after, by + 日期或年份, by then, until, by the end of, till, by the year 2022 等。

与将来进行时不同，将来完成时强调动作或事件在将来某一确切时间之前已经完成。而将来进行时则描述的是在将来某个确切时间正在进行或发生的动作。例如："By the end of the month, I will have completed my project." 指的是在月末前项目已经完成，而："By the end of the month, I will be working on my project." 则表示在月末前项目正在被进行。