A preposition is used before an object, noun, pronoun or a phrase, to form a phrase that describes a place, position, direction, method, time, object, aim, or the passive voice. Prepositional phrases can be used as adverbial adjuncts, objects, attributives or compliments. Normal prepositions that convey time such as “从”,“自”，“自从”，“由”，“打”，“在”，“当”，“于” are placed before time words. “当” is often used with “的时候” together to form an adverbial adjunct.

e.g. （1）他从6岁开始坚持每天锻炼身体。 （2）他自（自从）那天就再也没有迟到过。 （3）这家银行营业时间：由9点到17点。 （4）打昨天起他就觉得头痛。 （5）当我想起他的时候，他已经不见了。 （6）新中国成立于1949年。