余分: extra socks for a possible extra day and an unnecessary explanation
Distinguish an extra pair of socks packed for a possible longer trip from unnecessary explanation actually removed from a text.
An additional amount and an unnecessary part
余分 (よぶん) means extra or surplus. The sock sentence concerns an additional amount packed for a possible need; the text sentence concerns unnecessary explanation. Read what is extra together with its purpose.
余分に modifies packing an additional pair, while 余分な describes the explanation. These examples distinguish a useful reserve from a part to remove, without making everything extra wasteful.
The packed pair and the removed explanation
「予定より一日延びても困らないよう、靴下を余分に一足入れた。」 (I packed an extra pair of socks in case the trip lasted a day longer than planned.) An extra pair of socks was actually packed in case the trip lasted a day longer. 一日延びても gives the condition being prepared for. The sentence does not report an actual extension or use of the extra socks.
「文章から余分な説明を削った。」 (I removed the unnecessary explanation from the text.) The unnecessary explanation was actually removed from the text. 余分な説明 identifies the part removed. This result differs from preparing an extra amount for a possible need.
Read a reserve's purpose and an editing result
The sock sentence supplies the purpose of preparing for one extra day and the act of packing an extra pair. The explanation sentence supplies an actual removal. Identify what 余分 refers to and the action connected to it.
Packing a reserve and removing unnecessary material are different actions. The object and purpose help distinguish an extra amount that may be useful from a part identified as unnecessary.
How do the socks packed with 余分に and the removed 余分な説明 differ in their usefulness?
Show the answer and why
The extra pair was packed for a possible one-day extension, while the explanation was unnecessary and actually removed. The sock sentence does not report that the trip was extended or the extra socks used.