Lydianette Soza (asker) Jan 26, 2017:

Thanks Robert for the answer proposed and thank you Charles for the clarifications I'll make the respective changes.

Charles Davis Jan 26, 2017:

@Lydianette "Mayor dificultad" should be "most difficult" rather than "more difficult", and "para los estudiantes" should be "for (the) students" rather than "by students" (because the letters are difficult for them to pronounce). Alternatively, you could say "The letters that (the) students found most difficult to pronounce were...". You could either include the article before "students" or or omit it, depending on whether you want the sentence to refer to the particular students taking these tests or students in general.