6:45 in English can be said as "six forty-five" or "a quarter to seven." This is because in English, we often express time using the twelve-hour clock, where each hour is divided into 60 minutes. For example, 6:45 is 45 minutes before 7:00, which is commonly expressed as "a quarter to seven" or "six forty-five."

Here are some examples of how you could use this phrase in a sentence:

"I need to leave for work by six forty-five in the morning." (我需要在早上六点四十五分前离开去上班。)

"Can you meet me for lunch at six forty-five?" (你能在六点四十五分跟我约好吗？)

"The movie starts at six forty-five, so we need to leave soon." (电影六点四十五分开始，所以我们需要快点走。)

The way we express time in English comes from the way that time is traditionally measured and recorded using clocks and watches. In the past, people used sundials and hourglasses to measure time, but as technology advanced, more accurate timekeeping devices were developed, such as mechanical clocks and eventually, digital clocks. Today, we use a variety of timekeeping devices, but the way we express time in English remains largely the same.