What is the difference between
Do the two sentences have the same meaning? I'm trying to say:
I wrote this letter with your pen.
It's basically the same meaning. They both mean "I wrote this letter with your pencil" (not pen).
This one has "I" at the beginning.
This one doesn't have "I". That's ok. You can often leave out 私は in Japanese because it's clear from context. The other difference is, 手紙 became the topic of the sentence, so its particle を changed to は.