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1） 当主句是一般现在时或一般将来时时，从句可以用各种时态。
2）当主句是过去时态时，从句要根据动作发生的先后顺序，采取某一过去时态。
3 ）当主句中所表示的是科学真理、客观事实或格言时，从句必须用一般现在时。
从句里的动词时态与主句里的动词时态要一致的规律，需为时态呼应。
1 主句是一般现在时或一般将来时
当主句的动词是一般现在时或一般将来时的时候，从句（主要是受词从句）中的动作可以用各种时态。
例子：
I know that Jack works in that factory. 我知道杰克在那家工厂工作.
She doesn’t knowwhen her mother left home. 她不知道她母亲是何时离开家的.
I'm sure that Jim will cometonight. 我确信吉姆今晚会来.
I'll tellhim that you came tonight. 我将告诉他你今天晚上来过.
2 主句是过去时
当主句的动词是过去时态，从句的动词要根据动作发生的先后顺序，采取某一过去时态。
2-1 如果从句的动词所表示的动作发生在主句的动词所表示的动作之前，从句要用过去完成时或一般过去时，用一般过去时的时候，从句中常有表示确定的过去时间副词。
例子：
He asked me if Ihad been to London. 他问我是否去过伦敦.
He said his son was bornin 1977. 他说他的儿子在1977年出生.
2-2 如果从句中的动词所表示的动作与主句的动词所表示的动作同时发生，从句要用一般过去时或过去进行时。
例子：
I asked him whether he knew Japanese. 我问他是否懂日语.
The boy told me that he was looking forhis football. 那个男孩告诉我，他正在找他的足球。
2-3 如果从句的动词所表示的动作在主句的动词所表示的动作之后发生，从句的动词要用过去将来时。
例子：
I asked the teacher when we would have the exam. 我问老师我们将在什么时候考试.
I knew that Jane would comea few days later. 我知道过几天珍要来.
3 主句表达的是不受时间影响的真理
当主句中所表示的是科学真理、客观事实或格言时，即使主句的动词是过去式，从句的动词也必须用一般现在时。
例子：
He said that the moon is much nearer to the earth than the sun.
他说月亮比太阳离地球近得多.
When she was a child, she knew that Yu Shan is the highest mountain in Taiwan.
她从小就知道玉山是台湾最高的山
He told his son two plus two isfour.
他告诉儿子，二加二等于四。
4 容易犯错的句子
4-1 他进来时，我正在读书。
I read a book when he came in. ×
I was reading a book when he came in. √
句中有一个时间状语从句，表示主句的动作在过去的某一时间内正在进行，所以用过去进行时。
4-2 火车刚到达。
The train has arrived just now. ×
The train has just arrived. √
just（刚刚）常与现在完成时连用，放在助动词have/has和过去分词之间，just now（刚刚、刚才）则通常与一般过去时连用。例如：I arrived here just now. 我刚刚到这儿。
4-3 他加入俱乐部已有两年了。
He has joined the club for two years. ×
He has been a club member for two years. √
He joined the club two years ago. √
It is two years since he joined the club. √
join是非延续性动词，不能与表示一段时间的副词连用。
4-4 对不起，我没有看见你。
Sorry, I haven't seen you. ×
Sorry, I didn't see you. √
根据句意，是刚才没有看见对方，现在已经看见了，故用一般过去时。
4-5 他去过纽约。
He has gone to New York. ×
He has been to New York. √
have/has gone to表示“到某地去了”，目前还没有回来，have/has been to则表示“到过某地”，现在已经回来了。
4-6 我感冒两天了。
I have caught a cold for two days. ×
I have had a cold for two days. √
I caught a cold two days ago. √
It is two days since I caught a cold. √
catch a cold中catch 是非延续性动词，不能与表示延续性的时间副词连用。
4-7 我原不知道你在这里。
I don't know you are here. ×
I didn't know you were here. √
根据句意，说话时已经知道了，所以应用过去时态didn't know根据时态呼应的原则，宾语从句中的动词也用过去时态。
4-8 他说他两个月前就知道这件事了。
He said that he knew about it two months ago. ×
He said that he had known about it two months before. √
宾语从句中的动词所表示的动作发生在主句的动词所表示的动作之前，应用过去完成时，ago应改为before。
4-9 他上个星期就读完了这本书。
He had finished reading the book last week. ×
He finished reading the book last week. √
last week是表示过去某一特定的时间副词短语，应用一般过去时。
4-10 月球绕地球转。
The moon is turning around the earth. ×
The moon turns around the earth. √
表示客观存在或客观规律，常用一般现在时态，不用现在进行时。
4-11 我来这所学校已有两年了。
I have come to this school for two years. ×
I have been at this school for two years. √
come为非延续性动词，不能同表示一段时间的副词连用，应用be动词表示状态。
4-12 汤姆离校已有两周了。
Tom has left school for two weeks. ×
Tom has been away from school for two weeks. √
leave为非延续性动词，不能与表示一段时间的副词连用，应用表示状态的has been away from来表示。
4-13 你在哪里出生？
Where are you born? ×
Where were you born? √
“出生”指过去的情况，应用过去时态were。
4-14 如果明天不下雨的话，我们将去公园。
We will go to the park if it won't rain tomorrow. ×
We will go to the park if it doesn't rain tomorrow. √
在以if引导的条件状语从句中，如果说主句的动词是一般将来时，则该从句用一般现在时代替将来时，这种用法同样适用于以when，as soon as等引导的时间状语从句。I'll tell him the news as soon as he comes back. 他一回来，我就告诉他这个消息。
4-15 明年我将18岁。
I'm going to be eighteen next year. ×
I will be eighteen next year. √
“be going to＋原形动词”常用来表示计划，打算要做的事情，有时可与 “will 动词原型”互换使用，但若表示单纯的将来时，应用“will 原形动词”这一形式。
4-16 她正穿着一件红大衣。
She is having on a red coat. ×
She is wearing a red coat. √
She has on a red coat. √
have on不能用于进行时，可用一般现在时来代替，wear用于现在进行时或一般现在时均可以。
4-17 他们不在家里吃午饭。
They haven't lunch at home. ×
They don't have lunch at home. √
have lunch（吃午饭）中的have是实义动词，其否定式应在其前面加don't，不能用haven't。
4-18 听！鸟在唱歌。
Listen! The birds sing. ×
Listen! The birds are singing! √
Listen（听） Look（看）这样的字在句中表示动作已在进行，动词应用现在进行时。
4-19 看那些云，天要下雨了。
Look at those clouds, it will rain. ×
Look at those clouds, it's going to rain! √
根据主观判断表示将要发生的动作或状态，要用“be going to 原形动词”结构。不用"will 原形动词"结构。
4-20 他愿意和我一起去那儿。
He is going to there with me. ×
He will go there with me. √
含有意愿色彩的将来时不能用“be going to 原形动词”，要用“will 原形动词”结构。
4-21 他们什么时候到达的？
When have they arrived? ×
When did they arrive? √
疑问词when不能与现在完成时连用，可用一般过去时或一般将来时。该句是表示过去的动作，应用一般过去时。
4-22 昨天这个时候你在做什么？
What did you do this time yesterday? ×
What were you doing this time yesterday? √
表示过去某一时刻或某段时间内正在进行或发生的动作，要用过去进行时，过去进行时常与表示过去某一时间的副词连用。
4-23 到上学期末，我们已经学了500个英文单词、
By the end of last term, we learned 500 English words. ×
By the end of last term, we had learned 500 English words. √
表示在过去某一时间或动作之前就已经发生或完成的动作，要用过去完成时。
4-24 我睡觉的时候，他走进我的房间。
While I slept, he came into my room. ×
While I was sleeping, he came into my room. √
While I was asleep, he came into my room. √
while 常表示较长一段时间或一个过程，多用进行时态表示这一动作或状态。该句中表示“睡着”可用be sleeping 或be asleep。
