に耐える / に耐えない【にたえる / にたえない】

に耐える after a noun means something stands up to it: it survives use, holds up under criticism, is good enough to be looked at closely. The nouns are few and fixed – しよう使用, ひはん批判, かんしょう鑑賞, じつよう実用 – plus a physical strain like こうおん高温. The kanji こたえる堪える is also common in this sense.

に耐えない

The negative says something is not up to that standard. After the dictionary form of みる見る, きく聞く or よむ読む, it means "too awful to watch, hear or read".

The verb is in the dictionary form: ×見に耐えない.

Feelings: に堪えない

After a noun for a feeling, かんしゃ感謝にたえない堪えない and どうじょう同情にたえない堪えないmean the feeling is more than you can hold in. In this sense the standard kanji is 堪, and it is covered under にたえない堪えない.

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All of these are formal and written. In conversation, つかえる使える, みていられない見ていられない or がまん我慢できない say the same thing.