It's from my boyfriend, if you must know.

倘使你一定要知道的话，这是我男朋友给的。

《牛津词典》

You must know him—he's an institution around here!

你一定认识他—他是这一带的知名人士！

《牛津词典》

It scared the hell out of her, if you must know. And me, too.

这把她的魂都吓飞了，如果你一定要知道的话。而且我也被吓坏了。

《柯林斯英汉双解大词典》

"You know my rule,Andy. In all illegal activities, we must obey the law, in every detail.

VOA: special.2010.04.24

And you must know, as I looked on, I saw those men in a short time make the former democracy " look like a golden age."

而我一路看来，所见这些人在短期内的作为,似乎使旧民主,看起来像是进入了全盛时代“

耶鲁公开课 - 政治哲学导论课程节选

In order to attend, you must know the date and the time. You can ask your friend, "When is your wedding?"

为了能够出席，你必须知道具体的日期和时间，你可以问你的朋友，“When is your wedding？”

When 课堂 - SpeakingMax英语口语达人

But if you must know how the race is going, ask the math geeks.

FORBES: The Election is Over, and the Math Geeks Won.

You must know which seeds of opportunity are not growing as fast as you thought.

FORBES: How Google Refined the Entrepreneurial Spirit by Applying Strategic Focus to Invent a New Industry

Francis says that before joining a social network to make sales, you must know your client base.

FORBES: How To Use Social Media To Make Sales