i-adjective
no match for; cannot compete with; powerless against
Expresses that someone or something is overwhelmingly superior, so you cannot rival or defeat them. Often used with には or に.
彼にはかなわないよ。
I'm no match for him.
この暑さにはかなわない。
I can't handle this heat.
i-adjective
cannot stand; unbearable; intolerable
Used after an adjective in the て form (〜てかなわない) to mean 'so ... that I can't stand it'. Expresses strong emotional or physical discomfort.
暑くてかなわない。
It's so hot I can't stand it.
うるさくてかなわない。
It's unbearably noisy.
i-adjective
Dated usage meaning 'cannot do' or 'impossible'. Rare in modern Japanese; usually replaced by できない or other expressions.
昔の言葉で「かなわない」は「できない」という意味だった。
In old language, かなわない meant 'cannot do'.