现在完成时（Present perfect tense）
already, yet, now, just, by this time, etc.例如：
He has finished a new novel recently.
He has gone to London.
注意：just 与现在完成时连用， 而just now 与一般过去时连用，例如：
He has just received the letter.
He received a letter just now.
2．表示过去已经开始，持续到现在，而且还将持续下去的动作或状态。往往和表示一段时间的状语连用：today, this week, lately, recently, these days , during /for the last/past few years, since, for ages, etc. 例如：
We have had fine weather for the past few days.
He hasn’t come and seen me since then.
（1）瞬间动词的肯定式通常不与since, for 连用。常见的动词有：become , buy, borrow, bring, begin, break, come, die, finish, go , give, join, leave, lose, kill, marry, open, receive, start, stop, take, etc.
It is five years since he went to Canada.
He has been in Canada for five years.
He went to Canada five years ago.
Five years have passed since he went to Canada.
但瞬间动词的否定式可以和since, for 连用，例如：
He hasn’t left home for a month.( 表示“足不出户”的状态)
We haven’t seen each other since we parted.( 表示“少见”的状态)
What have you been doing this morning?(今天上午你一直在做什么？)
What have you done this morning?(今天上午你干什么了？)
3．完成时表示过去有过一段经历，这种经历不可能被以后的行动所抹杀或否定。常用时间状语有：often, never, ever, once, twice, before, since, etc.例如：
Have you ever heard her speak in public?
He has been to the library.
have been to 表示去过某地；have gone to 表示去了某地，不在此地。
We shall start at ten if it has stopped raining by that time.
I shall go as soon as I have finished my lessons.