名人教英文：preposition 介系字
prepositions are the most frequently used words in the english language. yet, they are the most difficult. this is because their usage is often governed by convention, rather than by hard and fast rules. once wrongly used, the preposition changes the meaning of a sentence altogether.
介系字是最常用、也是最难用的英文字。这是因为它的用法往往是跟习俗而不是跟严厉之规则，一旦用错，整句的意思也变了。
a preposition governs the relationship between its object and the words that come before it. it acts as an adjunct, the right choice of which gives an adjective, noun or verb more grammatical versatility.
介系字是用来支配其受事者与其前辞之关系。如同修饰语，可令其形容词、名词或动词在文法上多元化。
prepositions work with :
1. adjective
e.g. i am grateful for your help.
2. noun
e.g. the weather in hong kong is humid.
3. verb
a) to show different meanings of the same verb:
e.g. i look for my lost book. 我找寻遗失的书。
e.g. i will look into it. 我会注意此事。
e.g. look out! 小心﹗
b) certain prepositions must be used with certain verbs. 某些介系字必须跟某些动词一齐用︰
e.g. this book belongs to me. 这本书是我的。
e.g. this class consists of boys only.
这班只有男孩子。
e.g. carry on! 继续吧﹗
c) different prepositions increase a verb's grammatical versatility. 不同的介系字可令某动词变化良多︰