already（肯定）, yet（否定，疑问）, just, before, recently,still, lately,never,ever, never, twice, on several occasion,in the past few days/weeks/months/years,(up to)these few days/weeks/months/years, this morning/week/month/year, just, up to present, so far,up to now,till now,since+时间 这些都是关键词