Kali M. answered 04/08/19
Russian Language Tutor
Hi!
This is an excellent question. As I'm sure you know, the Russian language is quite different from the English language and requires you to change the way you think in order to form cohesive sentences fluently in Russian. Perhaps the largest difference between the two languages is the use of cases in Russian.
The reason each of these sentences is interchangeable with the other is because of the cases being used. You'll notice in the sentence "Я был не уверен," there has been a case applied to the word уверен (the infinitive is уверенный). Because there's an active case, it doesn't really affect the sentence if you move не around. The same with the second sentence. In English, a negative has a specific place in the sentence and you'd sound insane if you moved it around. Not so in Russian.
I hope this helps!
-Kali
Russian Language Tutor