I keep mixing up these two grammar points:
How can I decide that I should use one or the other?
For example in this sentence:
彼の生い立ちを聞いたら、同情せずには居られない。
Hearing his story, I can’t help but feel sympathy for him.
Why can’t it be 彼の生い立ちを聞いたら、同情してならない。?
From the intention (feeling a feeling because of X), it seems extremely similar to this example from “About てならない”:
ここ５年間娘と連絡がつかないから、心配でならない。
I can’t help but to feel worried because I haven’t heard from my daughter in 5 years.
I could be wrong about this, but I’m pretty sure you can use them somewhat interchangeably with some nuance difference. The てならない description mentions it’s used with emotions more than actions and this doesn’t seem the case with ずにはいられない-perhaps that’s the main difference? I’m sure someone will correct me if wrong lol.
I could be wrong, but it might have to do with this (from the てならない grammar point):
It’s just a guess though.
I also notice that ないではいられない and ずにはいられない are seemingly always about someone doing or feeling someway, not about describing another object.
Like with your image you posted for てならない, it says the reason why この漫画はつまらなくてならない can’t be used is due to it being momentary. But I don’t think it could be a ないではいられない sentence either as its describing the state of the manga not your own action.
Maybe I am wrong about this, but I would take ずにはいられない to be saying “if not doing A, I couldn’t exist” and so is naturally always going to be about the speakers own actions because it needs a verb. But with てならない it does not require a verb and can describe states. Like in " 愛知県の夏は暑くてならない"
So I interpret てならない as “It’s A, and it can’t/wont become anything else” which then becomes, very or if its about yourself “I can’t help but __”
So I think the only time these become interchangeable (maybe?) is when you’re using a verb to describe your own feelings or judgement.
Curious if anyone disagrees or has any corrections in what I said, as its how I have been thinking about it and has helped me so far.
To answer your question, I don’t see why it can’t be that either, I think it can.
同情せずには居られない = If not sympathizing (with him) I can’t exist.
心配でならない 。= It is concern and it won’t become anything else
I think 同情してならない Is also perfectly fine too but I could be wrong.
