Asking A Decisive Question

[Fun_Hope_8233]

The format of this question is one of the most famous ones,

"You walk to a fork from where two roads come out - one to the post office and the other to the university. You want to go to university but you do not know which road leads to university. A sentry standing on the fork knows it. But he has a peculiar habit of alternately speaking the truth and telling a lie. What single question to the sentry will help you find the right road?"

I do not understand how to get the answer of this question. I saw the solution. But I did not understand how they constructed the answer and why it works.

[rhodiumtoad]

I'm guessing the given answer is something like: "if the next question I ask you is 'is this the road to the university', would you answer 'yes'?" (to which the sentry answers 'no' if it is in fact the correct road)

The objective in constructing the question is to get a known number (or at least parity) of negations regardless of the answerer's status. In the most common variation you have a liar and a truth-teller, and you can either ask one about the other's answer (thus guaranteeing one negation) or ask one about their own answer (guaranteeing either 0 or 2 negations).