过去完成时是一种英语时态，用来描述在过去某个时间点或动作之前已经完成的动作或状态。以下是一些常见的过去完成时的标志词：
By：
表示“到...为止”，用于表示某个动作在过去某个时间点之前已经完成。例如：“I had finished my homework by 5 o'clock.”
Before：
表示“在...之前”，用于表示某个动作在过去某个时间点或事件之前已经完成。例如：“I had finished my homework before my mom got home.”
By the time：
表示“到...的时候”，用于表示某个动作在过去的某个时间点之前已经完成。例如：“By the time I arrived at the party, everyone had already left.”
Until：
表示“直到...”，有时与否定句连用，表示某个动作在过去的某个时间点之前已经完成。例如：“I hadn't finished my homework until 10 o'clock.”
Ever since：
表示“从那时起一直”，用于表示从过去某个时间点开始一直持续到现在的动作或状态。例如：“I had been studying English for five years ever since I was a child.”
Never before：
表示“从未”，用于强调某个动作在过去某个时间点之前从未发生过。例如：“I had never traveled outside of my hometown before this trip.”
Already：
表示“已经”，用于强调某个动作在过去某个时间点之前已经完成。例如：“I had already eaten breakfast before my friends arrived.”
Just：
表示“刚刚”，用于强调某个动作在过去某个时间点之前刚刚完成。例如：“I had just finished my homework when my friends called.”
Up until then：
表示“直到那时”，用于强调某个动作在过去某个时间点之前已经完成。例如：“I had lived here up until last year.”
By then：
表示“到那时为止”，用于强调某个动作在过去的某个时间点之前已经完成。例如：“By the time we arrived at the station, the train had already left.”
By that time：
表示“到那时为止”，用于强调某个动作在过去的某个时间点之前已经完成。例如：“By that time, I had already completed the project.”
这些标志词可以帮助我们更好地理解和构造过去完成时的句子，使句子的意思更加清晰明确。