1. The preposition
The use of verbs in English sentences is restricted by grammar rules: usually one sentence can contain only one predicate. If a sentence contains more than one verb, in most cases verbs appear in different levels, that is, the verb expressing the major action or state is used in the primary structure (the main clause) and the rest in the secondary structure (subordinate clauses of infinite forms like the infinitive or participles) unless as parallel joint by conjunctions.
Though belonging to function words, the preposition is a very useful part of speech, which not only clearly shows the relation between different parts of the sentence, but also frequently implies the notion of action or state. Then it can also be considered as a kind of secondary clause. So it is often translated into Chinese verbs. See the examples in the below.
- She is over some paper work. (那是她正忙于一些书面工作。)
- The dog is on the chain. (狗用链子锁着。)
Of course, in many cases the preposition or prepositional phrase can also be turned into verb when it serves other functions in the sentence. See the examples in the below.
- He has someone behind him. (有人给他撑腰。)
- She inspected the table for dust with her fingers. (她用手摸了摸桌子，检查是否有灰尘。)
Very often the preposition itself implies an action. therefore, when translated into Chinese, it’s better to be translated into a verb. For example, “he was reluctant to accepted the plan, and with reason” is better to be translated into “他不太愿意接受这个计划，他这样做是有道理的。”
2. The adjective
“The buses are crowded” can be well translated as “公共汽车很拥挤”, but it is also Ok to translated as “公共汽车上挤满了人”. The latter one sounds more natural because the dynamic verb “挤” makes the sentence more vivid.
The comparative degrees of adjectives (or adverbs) often imply the changes of situations or conditions, such changes can be better expressed by verbs (sometimes by adjectives that serves as predicates) in Chinese. For example, “my suit fit looser every day” suggests the Chinese saying of “衣带渐宽”.
Nothing is absolute in the world and there is exception to every rule. We say English verbs have a higher frequency than Chinese verbs, and English nouns are often translated into Chinese verbs.