... but the parts that are good are not original, and the parts that are original are not good.
... pero esos partes que son buenos no son originales, y esos que son originales no son buenos.
Very good indeed! Just a small correction = gender. And you are talking about the parts and not those parts, which is why "las" is a better option than "esas".
...Pero las partes que son buenas no son originales, y las que son originales, no son buenas.
(Note to self - check the dictionary more carefully; the gender of a word can differ depending on the meaning.)
Probably, the purpose was to make the meaning as clear as possible. Su can refer to el, ella, usted, ellos, or ellas.
isaid......the thesis from him is good and original ......why was this marked wrong?
Why is "the thesis good and original" not accepted? La does mean the not usually His
You omitted indicating whose these is good and original. The literal translation is, 'The thesis of him...', but this is not how it is said in English, so you have switch it around and say 'His thesis...'