19. Formation of Plural Nouns 

If a word ends in an -ción, -tad, -dad, or -tud, it'll be feminine. Also, if you are talking about a female, you will use the feminine articles (la señora). Words ending in an -o are masculine. Days of the week are also masculine. And if you're talking about a male, use the masculine articles. There are some exceptions to these rules and you just have to memorize them. To make words ending in a vowel plural, add an -s (libro = libros). Add an -es to make words ending in a constinant (papel = papeles). If a word ends in a -z, change the z to a c and add -es (cruz = cruces). After making the word plural, you must make the articles plural also. There are very few exceptions to making words plural.