求enough to,too to,so that的区别

在一般情况下,too...to...结构意为“太……以致不能……”,too后接形容词或副词,to是动词不定式符号,后接动词原形,动词不定式表示否定意义.但是,too...to...结构在以下情况中,动词不定式表示肯定意义.
一、too...to ...的意义
too...to...句型形式是肯定的,但表达否定意义,表示"太……而不能……".too的后面接形容词或副词的原级,to的后面接动词原形,构成动词不定式.too...to...句型是简单句.例如：
He is too young to join the army.他年龄太小,不能参军.
二、动词不定式的逻辑主语
动词不定式所发出的动作,其执行者与整个句子的主语不同指一个人或物时,需要在动词不定式前加上一个逻辑主语(for sb.).例如：
The maths problem is too difficult for me to work out.这道数学题太难我不能解出它.
三、too...to...结构表示肯定意义的情况
1.too前面含有表示否定意义的词,例如:not,never,nothing等时,too...to...结构不表示否定意义.例如：
One is never too old to learn.活到老,学到老.
2.如果在too...to...结构的前面出现了only,不仅免去了其否定意义,反而加强了它的肯定语气,only too相当于very或very much.例如：
I shall be only too pleased to get home.我将极高兴地回家.
3.too后为表示情感的形容词,例如：glad,pleased,happy,sad等时,表示的是肯定意义.例如：
He is too sad to hear the bad news.听到不幸的消息他太悲伤了.
四、too...to...结构与...enough to...结构及so...that...结构的相互转换
1.将too...to...结构转换为...enough to...结构时要注意：①enough前的形容词或副词须是too前面形容词或副词的反义词；②...enough to ...句式须用否定式；③too...to...结构有逻辑主语时,...enough to...结构也要加上逻辑主语.例如：
She is too young to do the work.→She isn't old enough to do the work.
2.将too...to...结构转换为so...that...结构时要注意：①so...that...结构是复合句,so的后面接形容词或副词,that的后面接从句；②that后面的从句要用否定形式.例如：
She is too young to do the work.→She is so young that she can't do the work.
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