8B Unit 1 Past and presentGrammar 现在完成时二、用法：现在完成时既涉及过去，又联系现在。
用法一：表示过去发生或已经完成的某一动作对现在造成的影响或结果。
常与just（刚刚），already（已经），never（从来没有），ever（曾经），before（以前），yet（仍然），once （一次），twice（两次），many times（很多次），how many times（多少次），so far（迄今为止），during the past（last）three years（最近三年来）等连用。
※副词的位置：①just常用于肯定句中，放在have / has后，He has just come .②never表示否定, 放在have / has后, He has never visited the Great Wall.③ever用于疑问句中,句型为: Have / Has＋主语＋ever＋过去分词？“…曾经……过吗？”用于询问某人过去的经历。
Have you ever been to the farm？④before用于句末，The woman has never heard of that before.⑤yet 用于句末或not 之后.Has the train arrived yet? No, not yet.⑥already用于肯定句, have / has 之后或句末.We have already finished it.⑦so far用于句首或句末. So far, we have visited the moon.用法二：表示过去已经开始，一直延续到现在的动作或状态。
时间状语有：①for＋表示一段时间的词语I have taught English for 19 years。
②since＋表示过去时间点的词语He has been at this school since 1986.③since＋表示过去的时间状语从句I have lived here since I was born.④since＋一段时间＋ago. I have known him since 20 years ago.三、现在完成时态中可以和表示一段时间状语（for，since，how long, all one’s life）连用的动词必须是表示延续的情况或动作的动词，即延续性动词。
如：be，have，know，live，work，study，learn，teach，keep，speak，talk，draw，wait，wear，walk，sleep，drive，write，do，clean等。
I have been a teacher for nearly 20 years. How long has he lived here?四、延续性动词的现在完成时可和包括“现在”在内的（到说话时仍未结束）表示一段时间的状语连用。
如this morning，today，this week，these days He has been to Beijing three times this year. He has written two letters this morning.（说话时间在上午）He wrote two letters this morning.（说话时间在下午或晚上）五、英语中还有一些动词的意义决定它们所表示的动作不能延续,只是一瞬间就结束的动作,这类动词叫做“非延续性动词”，常见的有：come，go，arrive，reach，see，hear，close，open，leave，begin，start，lose，buy，fall，join，die，get up，become，borrow，lend，find，finish，receive等。
这些动词可用于现在完成时，说明某个动作的结果还存在，但不能和表示一段时间的状语（how long，for，since）连用。
He has come back.（√）He has come back for two hours.（×）※但在否定句中，非延续性动词也可用表示一段时间的状语来修饰，如I haven’t heard from my father for a long time. We haven’t seen him since 1999.六、当终止性动词（非延续性动词）与表示一段时间的状语（how long，since，for，all one’s life）相矛盾时,改正错句的方法有如下几种:（1）用副词ago把现在完成时的句子改为一般过去时.He has come back for two weeks.（错）改为:He came back two weeks ago.（正）I have lost my bike for ten days. （错）改为:I lost my bike ten days ago. （正）（2）用“It is / has been＋时间＋since＋一般过去时态”句型来改写。
He has joined the League for 3 years. （错）It is 3 years since he joined the League.I have bought the book for 5 days. （错）It is 5 days since I bought the book. （正）He has died for 20 years. （错）It is 20 years since he died. （正）（3）用“时间＋has passed＋since＋一般过去时态”句型来改写。
He has left home for 20 years. 改为:Twenty years has passed since he left home.He has lost his pen for 2 days. 改为:Two days has passed since he lost his pen. （4）用系表结构来改写.He has died for 20 years. 改为: He has been dead for 20 years.The factory has opened since 1999. 改为:The factory has been open since 1999. How long has he left? 改为:How long has he been away?（5）用相应的延续性动词替代非延续性动词。
H e has bought the book for two weeks. 改为:He has had the book for two weeks.常见的相应转换形式如下:borrow / lend→keep, buy→have, finish / end→be over, arrive /come / go / move / reach /get to→be in /at / be here /be there, begin / start→be on , open→be open , close→be closed, die→be dead , leave→be away（from）,go to school→be in school / be a student, get up→be up, fall asleep→be asleep , fall ill→be ill, get to know→know, lose→be lost, become→be, return / come back / get back→be back, join→be in / be a…member, join the army→be in the army /be a soldier, receive / get a letter →have a letter , catch / get a cold→have a cold, begin to study→study,他参军已有三年.He has joined the army for three years.（错）改为:He has been in the army for three years / since three years ago.He has been a soldier for three years / since three years ago.He joined the army three years ago.It is three years since he joined the army.Three years has passed since he joined the army.（1）He came to our village two years ago.=He our village since two years ago.（2）He left home three days ago=He home for 3 days. （3）I bought the watch 2 weeks ago=I the watch since 2 weeks ago. （4）It is 5 days since I borrowed the book=I the book for 5 days. （5）The film has begun= The film for half an hour.（6）I got to know him 10 years ago=I him for 10 years.（7）There is a factory=There a factory for 20 years.（8）Our school opened in 1960=Our school since 1960.七、现在完成时中应注意的几个问题1. have been to 和have gone to的区别have been to 强调“去过”，现已不在那里，如：He has been to the USA three times.他到美国去过三次。
（过去“到美国”，现在已“不在美国”）have gone to主要强调的是“去了”，现在人不在说话的现场，如：--Where's your mother? --你妈妈在哪？--She has gone to the hospital. --她去医院了。