Thank you both for answering!
I just thought that the "I think" part was used to express an opinion, in the same way as we use "I guess", "I suppose", etc. That's why I expected something after "therefore I am".
Today I had an English class where I had an argument with a teacher, so I wanted to ask you about it. I had written an annotation for a research article (as prescribed by my study plan) and she checked it today and said a couple of things were wrong and tried to explain to me why. So, I've wanted to debunk her ideas because I intuitively feel that I'm correct, yet I can't express it in terms of grammar, nor do I have authority of being a native English speaker.
The first sentence I'd like to ask about is "This type of behavior is not appropriate for neural network simulation where millions and sometimes billions of iterations need to be applied to a structure."
The teacher marked the word "need" in red. She said that in this context only a passive form of "need" could be used, i.e.: "Billions of iterations are needed to be applied to a structure", or that I should have used a different verb instead of "need". She said that "are" is the best option in the given context and some other verbs are possible, too, like "should" and "must". Since I was writing the annotation myself, I used "need" and not any other verb for a reason. I just want you to tell me if it's correct as is. I suspect this has something to do with the middle voice, but I'm not sure.
The second sentence was "Cycle after cycle, this force slowly stretched the nanoparticles out until they smeared into a very thick uniform film of single vanadium pentoxide molecules without the potential energy centers which completely inhibited its ability to conduct, as ? was observed on a voltmeter screen" where she put a question mark before the word "was" in the second part. I'm not sure what she actually meant there, so I just want to know if such phrasing is fine (without anything put in before the "was"). I'm struggling to recall which grammar topics I should read on it, but I feel this is correct anyway, though I'm not totally sure now that it's been corrected. (I even think that in a lower register the "was" could be dropped off completely and we'd still be left with a legitimate sentence, with a little exception that the fact that the observation took place namely in the past would now be a bit unclear).