衰弱

1. Origins, structure, and basic meaning

• Structure: 衰弱 (すいじゃく) means severe weakening of the body or vitality. 衰弱していた is an established state; 衰弱していった is progressive change.
• Basic usage: A horse is already debilitated after a long journey, while a fictional old man grows weaker day by day.

2. Common pitfalls and similar expressions

• Accumulated weak state and ongoing decline
  • していた presents the continuing condition at the observation point; していった traces change over time. Neither should be reduced to a momentary completion.