Dear Zivanai,
As far as your sentence is concerned, I see two options turning it into its negative form:
1) He did not take a huge risk sending somebody experienced.
(in this case, I have to change 'inexperienced' to 'experienced' so that the sentence still makes sense)
2) He took a big risk not sending somebody experienced.
(this is the other option, but I also have to turn 'inexperienced' into 'experienced' for the sentence to make sense)
In grammar questions, they often want you to turn somebody (affirmative) into anybody (negative), but 'anybody' does not make any sense here. We cannot blindly turn 'somebody' into 'anybody'; it still needs to make sense.
I hope this helps!
