Question

这个句子里为什么不用You'd better don't look it up at once而用not look it up ? When you come……

When you come across a new word while reading, you'd better not look it up at once.
上面这个句子我有两个问题：
1）为什么不用You'd better don't look it up at once而用not look it up ?
2）为什么不是not to look it up ?
谢谢！

Answer 1

因为had better相当于一个助动词或者情态动词,它的后面要跟动词原形。用法如下：

口语中常省略成 'd better或better。如：

 你最好小心点。 

正：You had better be careful.  

正：You’d better be careful.  

正：Better be careful.  

■注意否定式、疑问式及其回答。如： 

You’d better not go there. 你最好不要去那里。 

He’d better go with us, hadn’t he? 他最好同我们一块去，对吗？ 

What had we better do? 我们最好做什么？

 Had we better not go? 我们不去好不好？ 

比较：Hadn’t we better go? 我们去不是更好吗？  

■可接进行式、被动式或完成式。如：

 I’d be going now. 现在我得走了。(心情近切)  

She’d better be met by him. 最好他去接她。 

You’d better have done that. 你要是把那件事办了就好了。

 ■注意： 

1. 此结构中的had永远不可改为have或has； 2. 对上级或长辈不宜用此结构。 

   
   

Answer 2

"had better do something"就是一个惯用词句 意思是 你最好干嘛干嘛

一般非正式的会把had缩写成 'd，比如You'd better look it up, 或是直接省略掉 比如 You better look it up.
否定就是加个not，不需要改变它原来的结构。所以是You'd better not look it up.

Don't do something 一般是祈使句，直接用在句子开头