There are two endings to make plural of adjectives and nouns in Spanish. There is a plural case when the word does not change from the singular, however this is not common. The election of one of these options must adjust to the following rules:
a) Nouns and adjectives ending in vowel –atona or in –e tónica*, makes the word plural with -s.
Casa – casas // estudiante – estudiantes
b) Nouns and adjectives ending in -a or in -o tónicas. The plural is formed with -s, e.g., papá – papás // sofá – sofás. There are some exceptions like the adverb ‘no’ which would be ‘noes’ in plural, adding -es.
c) Nouns and adjectives ending in -i or in -u tónicas. This use usually accepts both endings, -es and -s, although -es is used for formal language.
Bisturí – bisturíes (or bisturí) // Tabú – tabúes (or tabús)
In the case of demonym (where someone is from), we more commonly use the option -es. I.e. marroquí – marroquíes; and the words that come from other languages, usually are formed with -s, e.g., champú – champús.
As in the case of the adverb ‘sí’ the plural would be ‘síes’.
d) Nouns and adjectives ending in -y with vowel before. Most often, plural is made with -es. I.e. rey – reyes // ley – leyes. However, some modern words, for example the words added from other languages in the last years have an ending with -s, in which case ‘y’ changes to ‘i’ when plural, e.g., jersey – jerséis // espray – espráis.
e) Foreign words ending in -y with consonant before. In order to adapt these words to Spanish form, the ‘y’ is transformed into ‘i’ and the plural made with -s, e.g., dandi – dandis (English: dandy); ferri – ferris (English: ferry).
f) Nouns and adjectives ending in -s or in -x. There are two scenarios: if word is one syllable or stressed polysyllabic (has the accent), the plural form adds the -es; second option, rest of words then the plural does not change its form from singular.
1.Example: tos – toses // compás – compases 2. Example: crisis – crisis // tórax – tórax
g) Nouns and adjectives ending in -l, -r, -n, -d, -z, -j. When there is a vowel before, it forms its plural with -es, e.g., color – colores. Same applies for foreign words, e.g., máster – másteres.
h) Nouns and adjectives ending in consonants other than the previous ones. They make plural with -s.
i) Nouns and adjectives ending in -ch. There are foreign words that can stay with the same form or end in -es.
1. Name: (Los) Zarevich 2. Sándwich – sándwiches
j) Nouns and adjectives ending a consonant group. They are foreign words as well and make plural with -s, e.g., iceberg – icebergs // récord – récords
*Vocal atónica is the vowel which does not have the accent inside the word.
An interesting topic to learn following plural would be the gender in Spanish nouns, check this link!