完成时态是英语语法中的一种时态，用来表示一个动作或状态在过去发生并且已经结束，或者从过去某一时间开始一直持续到现在。完成时态的构成主要分为两部分：助动词和实义动词的过去分词-ed。
现在完成时
结构：主语 + have/has + 动词的过去分词
用法：表示过去发生的动作或状态，强调该动作已经完成；或者表示从过去某一时刻开始，持续到现在的动作或状态。
过去完成时
结构：主语 + had + 动词的过去分词
用法：表示在过去某一时间或动作之前已经发生或完成了的动作，即动作发生在过去的过去。
将来完成时
结构：主语 + will have + 动词的过去分词
用法：表示在将来某一时间点之前会已经完成的动作。
示例
现在完成时：
I have studied English for 5 years.
They have already eaten breakfast.
过去完成时：
By the time I arrived, they had already left.
She had finished her homework before going to bed.
将来完成时：
By next week, they will have completed the project.
By the end of this year, I will have saved enough money for a new car.
希望这些信息对你有所帮助。