Breakdown of Ellos han manchado la carta que he escrito.
la carta
the letter
ellos
they
escribir
to write
que
that / than
Questions & Answers about Ellos han manchado la carta que he escrito.
Why is the word for "written" escrito instead of "escribido"?
Escribir (to write) has an irregular past participle. Instead of following the normal rule of changing the -ir ending to -ido, it becomes escrito. This is one of a small group of highly common verbs with irregular past participles that you just need to memorize.
Since la carta is a feminine word, shouldn't it be escrita?
Good catch, but no! When you use a past participle with the helper verb haber to say what someone "has done" (like he escrito or han manchado), it always ends in -o. It only changes to match masculine/feminine or singular/plural if you are using it purely as a descriptive adjective, like in the phrase una carta escrita (a written letter).
Why are there two different helper verbs (han and he) in this sentence?
Can I leave out the word que like we often drop "that" in English?