"I can't bear to sit there for 10 hours" can be translated (literally) as "我（忍）受不了坐在那里10个小时”。 But is "could" here to express past tense or subjunctive mood? In the first case, you need to use an adverb, e.g. 当时 or 那时, to make the time clear. In the second case, as there's no subjunctive mood in Chinese, you need other adverbs or conjunctions to express assumption etc.

"I can't bear to sit there for 10 hours" can be translated (literally) as "我（忍）受不了坐在那里10个小时”。 But is "could" here to express past tense or subjunctive mood? In the first case, you need to use an adverb, e.g. 当时 or 那时, to make the time clear. In the second case, as there's no subjunctive mood in Chinese, you need other adverbs or conjunctions to express assumption etc.

您最好将这两个词放入具体语境来提问，这样我们比较容易明白你的意思

我无法忍受。。。

I am sorry, I do not know what you want to express, because I always say cannot bear sth or doing. My English is not very good, but since you refer to couldn't, I think you want to emphasize Polite tone maybe, but in Chinese, there is no obvious difference, 不忍心is enough 不忍心去做.... the latter one, 做......实属不忍啊。we always use 啊，哪to express our pity.