The modal verb 忍心
Modal verbs (情态动词) can be used to express possibility, desire, ability, and necessity. Modal verbs cannot appear without a predicate and can only appear in predicate phrases.
The verb 忍心 
emphasizes that a person is ready to do something.
The verb is often used in the negative form with the particle 不 placed before it.
emphasizes that a person is ready to do something.
The verb is often used in the negative form with the particle 不 placed before it.Examples
他们那么可爱，我怎么能忍心骗他们呢？
They are so nice. How can I dare to lie to them?
tāmen nàme kě'ài, wǒ zěnme néng rěnxīn piàn tāmen ne
我们一直都知道，但是不忍心告诉她
We’ve always known, but we didn’t dare to tell her
wǒmen yīzhí dōu zhīdao, dànshì bù rěnxīn gàosu tā
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