weak substitution

例句：

1. 1. The old man was weak from lack of exercise.
   老人由于缺乏锻炼而身体虚弱。
2. 2. The baby is too weak to walk yet.
   这个婴儿还太小，不能走路。
3. 3. The old tree is weak and needs to be replaced.
   这棵老树很虚弱，需要更换。