For the English speaker learning Spanish as a second language, most of these words pose little problem in spelling, as the differences between the two languages usually follow regular patterns. Below are listed the most common regular differences in spelling as well as a selection of words whose differences don't fit these patterns. The emphasis here is on words that are likely to cause spelling problems, not ordinary differences in the languages such as radio for the English "radium" and dentista for "dentist."
English "-tion" as the equivalent of Spanish -ción: Hundreds of words fit this pattern. The English "nation" is nación in Spanish, and "perception" is percepción.
Avoidance of double letters in Spanish: Except for recent words of foreign origin (such as express), the use of rr and, less commonly, the use of cc (where the second c is followed by i or e), Spanish generally doesn't use double letters in English cognates. (Spanish does treat ll as a single letter, but two of the very few English cognates that shares that usage are "llama" and "guerrilla," both of which were adopted from Spanish.) Thus the English "libretto" is libreto in Spanish, "possible" is posible, and "illegal" is ilegal. Examples of rr or cc in cognates include acción, acceso and irrigación. One Spanish word that doesn't fit this pattern is perenne (perennial).
Avoidance of k in Spanish: Except for a few Greek words (such as kilómetro and some words of other foreign origin such as kamikaze and various place names), Spanish cognates of English words with a "k" usually use a c or qu. Examples include quimioterapia (chemotherapy) and Corea. Some words are spelled both ways: caqui and kaki are both used for "khaki," and both bikini and biquini are used.
Simplification in Spanish: A number of words, particularly ones whose English spellings come from French, have more phonetic spellings in Spanish. For example, "bureau" is buró and "chauffeur" is chófer or chofer, depending on the region.