nowhere in structure does it say that theres it is used without に. It’s a new grammar for me, so that makes it very confusing
also what form is ついていなかった?
Thanks 
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Well 気がつく is a phrase in itself that it often connected with に so it’s normal that it can be used without it. In the first example the phrase is being by itself and in the second what is realized is marked by は which changes the focus to the fact you realized it (whereas に give focus to what was realized.
As for ついていなかった that is negative past progressive form (or something along those lines). Basically you were in a state of not realizing (aka “didn’t realize”).
Another thing it doesn’t mention is that the phrase can be used for when someone regains consciousness or wakes up.
ついていなかった
You’ll get there eventually but here’s a build-up:
つく — plain form
ついて — te form (for う-verbs)
ついている — progressive/state form (te form + iru) (iru is る-verb)
ついていない — negative progressive/state form
ついていなかった — negative past progressive/state form
がんばってね！
Thank you guys. it would be helpful if it was explained in ‘structure’ that に doesn’t need to be used in some situations? For you experienced vets it is probably second nature. Just confusing for us just learning 
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I’d say you have a valid point here, and maybe one of the moderators will weigh in?
@Pushindawood @Jake
For my two cents, I’ll offer this. Particles like に,の,が,と are used in lots of expressions and it’s just one of the quirks of Japanese grammar. As a whole, it probably doesn’t make sense to explain on every grammar point that the particle is not always needed, but it would be good to have a discussion about it somewhere early in the journey. In this case, the particle に is the way the verb/noun in front gets connected to phrase in back.
I didn’t start from scratch on this site so I don’t have the perspective that you have. I was actually taught that the particles “go with” the thing before it, so to be extra explicit with the grammar point, I might write “Noun + に + 気がつく”. But again, I understand the designers glossing over this point to make a cleaner presentation and because it’s so common.
(p.s. There’s a lot of quirks in Japanese. It’s best to just accept it and know that it’ll make more sense as you see more examples of it.)
Cheers!
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I always hated this particular grammar, it never seemed to stick with me.
「気がつく」は 付いていないの？
(The grammar is not sticking?)
付く= to stick, to attach
Sorry, I couldn’t resist making a pun. 
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On that topic, do you know why we say ついていなかった instead of つかなかった?
@MrMilosz Think about it like a timeline. At some point you realize X. Before that time you were in a continuous state of not noticing then switched to a continuous state of knowing. Thus,
ついていなかった → ついた → ついている
Similar to other “state” verbs the past tense (ついた) can represent the moment of instantaneous change.
If I had to guess, the rest go like this:
つく - will notice (when talking about the future)
ついた - noticed (when talking about the past)
つかない - won’t notice (when talking about the future)
つかなかった - didn’t notice (when talking about the past)
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I’ll add a couple of my observations (not a full answer to your question).
- instantaneous action verbs need ている to express state
私は覚えている – I remember (I am in the state of remembering)
私は覚える – I will remember
私はここにいる – I am here (“to exist” does not need te-iru) - when telling a story, ている gives a feeling of being there rather than just a dry statement of facts.
examples:
もう晩ごはんを食べた？
いいえ、まだたべていない。
No, I haven’t eaten yet. (No, I am not yet in the state of eating.)
きのう、晩ごはんを食べた？
いいえ、食べなかった。
No, I didn’t eat. (and the story is done)
きのうね、晩ごはんを食べていなくて腹が痛かった。
Yesterday, I didn’t eat (hadn’t eaten) dinner and my stomach hurt. (I’m feeling in the middle of the story)
…気がついていなかった – I didn’t realize… (in that moment)
…気がつかなかった – I didn’t realize… (simple, done, dry, don’t ask me)
(p.s. I’d love someone to provide more insight to this.)
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