I struggle with these two expressions which mean two different things... especially knowing that the は can be omitted.
このケーキを食べて(は)ならない。(I) must not eat that cake.
このケーキを食べてならない。(I) cannot help eating that cake. I am dying to eat that cake.
Apart from the context, any cue on how to distinguish them ?
The first one, I see it, as "About eating the cake, wont do" thus, I must not eat it ! The second one, I dont really know how to deconstruct it.
Grammatically both are the same and should be considered as a set phrase. デジタル大辞泉 has an entry for this:
１ （「てならない」の形で）その事について抑えようのない気持ちを表す。「話の続きが気になって―◦ない」「おかしくて―◦ない」
２ （多く「てはならない」の形で）禁止を表す。「絶対に忘れては―◦ない」「許可なしに入室しては―◦ない」
#1 corresponds to your second example and #2 the first.
In terms of how to determine the usage, as stated in this and should be clear from the definition, "can't help" meaning is preceded by a word expressing feelings (emphasis mine).
感情や感覚の形容詞を使う。
一人称に使う。三人称の場合、文末が推量の形になる。
少し古い言い方。
動詞は自然にそうなる自発動詞が多い: 思える・思い出される・思われる・感じられる・泣ける・見える・気になる・悔やまれるなどです。
As such, your second example is not quite idiomatic.
