transformation

You must jog at least thrice a week, lest you are fit for the match.

"Thrice" is a dramatic, dated word.

"You must jog at least three times a week to be fit for the match.

"Lest" is also a dramatic, dated word and it doesn't make sense in the context you provide.

You must jog at least three times a week or you will be unfit for the match.

"Lest" could be substituted here: "You must jog...lest you be unfit..."
but dramatic, dated words dilute contemporary meaning.

Noddy, The problem with the question is how he/she can use "lest" in the sentence provided.

Yoong didn't ask for a reconstruction of his sentence.

C.I.--

Elsewhere Yoong Liat has asked for intensive correction and analysis. He wants to master contemporary English, not merely textbook English.

Noddy, Gotcha. I'm afriad my ability to read between the lines has always lacked the skills to interpret English properly.

C.I.--

I've read that the Chinese government is very enthusiastic about encouraging English language classes.

Unfortunately English taught By the Book by an Instructor who learned By the Book can be very dated and stilted.

Especially when they use textbooks from the late 19th century. LOL

Staying with Yoong Liat's question- "You must jog at least thrice a week, lest you be unfit for the match. I agree with those who have stated that the terminology is obsolete

I agree with your answer. We've to follow obsolete terminology in transforming the the sentence.

Thanks.

Yeah... I understand what the words "thrice" and "lest" mean. I would never use either of these words and I would think it strange if any of my friends used them.

I think he's using a textbook from the 19th century - unfortunately.

Why is 'twice' correct, but 'thrice' is dated? When did it happen and how? Please explain so that I can have a clearer picture.

Thanks.

Words just become obsolete through lack of usage. That is why they are termed obsolete rather than incorrect. Words such as "dasn't" meaning "dare not" and "shan't" meaning "shall not" were once used in everyday conversation. Now they are obsolete. Few on this board are old enough to have been around when they were in use.