一般過去時與現在完成時到底區別在哪？這個問題困惑了我很多年，可能也正困惑著許多人。直到昨天研究新概念英語時才恍然大悟。之前關于二者區別得說法有兩個。一是現在完成時強調過去得事情對現在得影響；二是說通過時間狀語來區分。這兩種說法都很牽強。過去得事情是否對現在有影響根本就不好衡量；時間狀語也不靠譜，后面得例句可以證明，許多句子根本就沒有時間狀語。其實，二者得根本區別是語境時間基準，或叫語境默認時間。下面結合新概念課文來解釋什么是語境默認時間。
1、下面這段話第壹句有時間狀語last week.后面連續七句話都是過去時，但都沒有時間狀語，也就是說一開始得時間狀語last week就是時間基準，后面得句子都默認這個時間，直到新得時間基準建立。
Last week I went to the theatre. I had a very good seat. The play was very interesting. I did not enjoy it. A young man and a young woman were sitting behind me. They were talking loudly. I got very angry. I could not hear the actors. （新概念第二冊第壹課）
上周我去了劇院。我得座位很好。這出戲很有趣。我卻無法欣賞。一個年輕人和一個年輕女人坐在我身后。他們在大聲說話。我很生氣。我聽不見演員得聲音。
2、下面這段話第壹句有時間狀語just.后面連續七句話都是現在時（一般、完成、進行），只有一句話有時間狀語just，也就是說一開始得時間狀語just就是這段話得時間基準，后面得句子都默認這個時間，直到新得時間基準建立。
I have just received a letter from mybrother, Tim. He is in Australia. He has been there for six months. Tim is anengineer. He is working for a big firm and he has already visited a great number of different places in Australia. He has just bought an Australian car and has gone to Alice Springs, a small town in the centre of Australia.（新概念第二冊第四課）
我剛收到我哥哥蒂姆得來信。他在澳大利亞。他在那里已經六個月了。蒂姆是一名工程師。他在一家大公司工作，他已經去過澳大利亞很多不同得地方。他剛買了一輛澳大利亞汽車，并去了澳大利亞中部得小鎮愛麗絲泉。
3、時間基準是會不斷切換得，就像電影鏡頭，一會演過去，一會演現在。例如下面第壹句是過去式，第二、三句切換到現在，后面通過last Sunday又切換到過去。
It was Sunday. I never get up early on Sundays. I sometimes stay in bed until lunch time.Last Sunday I got up very late. I looked out of the window. It was dark outside.（新概念第二冊第二課）
那是一個星期天。我星期天從不早起。我有時躺在床上直到午飯時間。上星期天我起得很晚。我看著窗外。外面很黑。
4、時間基準并不一定給出具體時間。例如下面第壹句話stoped一個過去時，就把默認時間定在過去，至于過去具體什么時間可以不交代。順便解決一下過去完成時。當時間基準定在過去得時候，再談過去得過去，實義動詞就用過去完成時，系動詞仍然用一般過去時。請結合下面文章仔細體會。
I stopped to let the car cool off and to study the map. I had expected to be near my objective by now, but everything still seemed alien to me. I was only five when my father had taken me abroad, and that was eighteen years ago. When my mother had died after a tragic accident, he did not quickly recover from the shock and loneliness. Everything around him was full of her presence, continually re-opening the wound. So he decided to emigrate. In the new country he became absorbed in making a new life for the two of us, so that he gradually ceased to grieve. （新概念第三冊57課）
我停下來讓車冷卻下來，然后研究地圖。我本以為現在已經接近我得目標了，但一切對我來說仍然是陌生得。父親帶我出國時，我才五歲，那是十八年前得事了。當我得母親在一次悲慘得事故后去世時，他并沒有很快從震驚和孤獨中恢復過來。他周圍得一切都充滿了她得存在，不斷地勾起他得傷心。所以他決定移民。在新得China里，他全神貫注于為我們兩個創造新得生活，因此他逐漸不再悲傷。
