If I lose this pencil, no other pencil can replace it and the letter will remain eternally unfinished.
It is a very special pencil
May I know the mistake for having the order of words differently?↓
No mistake but not natural.
In this sequence you probably better use は after てがみ so the listener knows about the topic first.
And I will take this potato chip and eat it
Why is ペン wrong and 鉛筆 right?
Because ペン means pen and 鉛筆「えんぴつ」means pencil... I don't know if in your country the meanings are switched or if the names are similar but pen is generally the one with ink, and pencil is the one made from wood.
ペン is the Japanese word for "pen" while 鉛筆 (えんぴつ) is the word for pencil. The question asked for the latter.
Gosh you're so dramatic