unable to meet obligation

例句：

1. 1.He is unable to speak English fluently.
   他无法流利地说英语。
2. 2.The old man was unable to walk without help.
   老人无法不借助帮助行走。
3. 3. The car was unable to start due to a faulty battery.
   汽车因电池故障而无法启动。