表示一个从过去某时开始,延续到现在并可能持续下去的动作,常与表示一段时间的时间状语连用,如: so far, up till now, up to now, since, for a long time, up to present, up till present, in the past/in the last few years, these days, many times等。
用于表示第几次做某事,常与this/it is the first(second/third/last)time that和that/it is the形容词/副词的最高级+名词