"Is it" x "It is" em frases afirmativas

Taka a look a this sentence:

"(...) Like many other teachers, I discovered that not only is it hard for some students to solve (what seemed to me) simple problems by themselves, but it is also hard for them to understand the solutions that are given to them."

My question is about that "is it". Couldn't it be "it is"? When do I use "is it" in affirmative sentences?

Thank you in advance!