"Peter does not want the knife."

Translation: पीटर को चाकू नहीं चाहिए।

[Caukoyuki]

What use does को have in this sentence?

[vinay92]

The Hindi sentence would be literally translated as 'To Peter, the knife is not wanted'. को corresponds to 'To' here.

Sentences with चाहिए have to be constructed this way.

[Trevor192456]

I seem to remember there have been many sentences in the course where the word for "want" has been used without "ko/ka/ki". What could explain this? (I hope my meaning is understood as I cannot type Hindi with my phone)

[vinay92]

The को is necessary only with the चाहिए form.
Other forms of चाहना behave like any other verb and don't need the को.
I have explained the difference between using chahiye and other forms of chahna in a comment below.