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In Chinese, the character "自" (zì) is used to indicate the starting point of time or place. It functions similarly to "since" or "from" in English. When used to introduce a time frame, "自" is often followed by a specific date or event to denote when something began. For example, in the sentence "自1978年以来，中国发生了很大的变化," it translates to "Since 1978, China has undergone significant changes," indicating that the changes started in 1978 and continue to the present.
Similarly, "自" can also be used to indicate a starting location. In the sentence "我们的航班准时自北京出发," it means "Our flight departs on time from Beijing," where "自北京" specifies the place from which the flight is departing.
Overall, "自" is a versatile preposition that effectively marks the origin of time or place in sentences, providing clarity and context to the information being conveyed.