[mmyuyu]

Hi Eway,
Sorry again for the late reply. All these four words are prepositions. The proper usage of the first pair 對and 向 is 對 , a preposition for people and events, must be used with a specific object. For example, we say wo dui hai xian guo ming. ( I am allergic to seafood.)向 is a preposition for time and place and it means going in the direction of. For example, ta xiang xue xiao zou. ( He goes in the direction of the school.) Another pair is 由 and被 and they are both prepositions too.由is a preposition for time and place and it means starting or the beginning stage of doing something and it also means the end of doing something. For example, wo jin tian you zhe ye du qi. ( I start reading from this page today.) 被is usually used with a specific object and the verb used with bei must be a transitive verb.For example. wo bei mama chu fa. ( I was punished by Mom.) It is ususally used to present the passive voice. However, you might find native speakers use them in a mixed way. Language changes over time so we can not really say they are wrong when they say wo xiang ta shuo( I said something to him.) This happens to spoken language often and it is acceptable to natives.But grammatical? I hope I do answer your questions. Chinese is an old languag and some people use it in their own ways, especially spoken form.