One day,scientists will be able to find a cure for cancer. (NOT...will can find...)
后面的不定式通常不属于被动语态。
The mistake is able to be corrected. 该句应改作The mistake is capable of being corrected.
指能力时,不一定意味着实行,如“经过了两个月的步行,他们到达了目的地”通常不说 After a two-month tramp, they wereableto reach their destination.而是用 After a two-month tramp they reached their destination。