But also:

Il ne faut pas perdre ses clés. -- One must not lose one's keys.

Occasionally, il ne faut pas means "it is not necessary", but usually it means "must not", even if that might seem illogical. It's also the most natural way to say it in French.

The same is true for devoir, by the way.

Hi Rowena,

If we write "Il ne faut pas ......" this seems to me to mean  "It is not necessary ....."

The statement to be translated is "He must not lose his keys" so the negation needs to be on perdre does it not?

This is how I see it  --  hope it helps?

JIm

Il faut ne pas perdre ses clés. -- One must not lose one's keys.