You know that, as in English, we add an -s or -es to form the plural, so we have “perro, perros” and “animal, animales”. But we also have some words that are singular and end with an -s, and you already know some of them! Remember the days of the week: lunes, martes, miércoles, jueves, viernes. Now, I will teach you the rest, and also the logic behind this. We have basically five little groups:
Words that are identical in the singular and the plural: análisis, antítesis, apocalipsis, cactus, caries, conjuntivitis (and apendicitis, bronquitis, etc.), crisis, diabetes, dosis, eucaliptus, herpes, lapsus, neurosis, oasis, profilaxis, síntesis, tesis, virus, and the days from lunes to viernes. Notice that some of them change in English (cactus, cacti or cactuses; crisis, crises; analysis, analyses, virus and viruses), but not in Spanish. They remain exactly the same in the plural.
Words connected with a double item or things used on many occasions (identical in the singular and the plural):
Lavavajillas (dishwasher), sacacorchos (corkscrew), sacapuntas (sharpener), abrelatas (can-opener), parabrisas (windscreen), paracaídas (parachute), paraguas (umbrella), pararrayos (lightning conductor), rompecabezas (puzzle), trabalenguas (tongue-twister), are words which end with an -s because these things are singular items but they are used again and again. Same applies to cumpleaños (birthday), because it happens year after year.
Biceps, triceps and cuadriceps are muscles with latin names. The biceps has two parts, the triceps has three and the cuadriceps has four.
Then, we have objects like anteojos o gafas (glasses), pantalones o pantalón (trousers), tijeras o tijera (scissors). These objects are made up of two parts, so in the case of trousers and scissors, some people use the singular form and some others the plural, and both options are considered correct.
Uncountable Nouns which cannot be used in the plural: pis (familiar word for “pee”), pus (as in English, the yellow thicky and disgusting liquid when a wound is infected), tos (cough).
Formal words which are used always in the plural and preceded by the article: las nupcias (the wedding), las exequias (the funeral). We usually say “boda” and “funeral” for wedding and funeral, but “nupcias” and “exequias” are more solemn.
Words ending with -s in the singular which add -es to form the plural: notice that this is the only group which changes the form in the plural. As (ace), bis (encore), jamás (never. It is like “nunca”, but more emphatic, something like “never in the life”), marqués (marquess), compás (bar in music or the object to measure angles. It is not a compass to know where is the north, this is called “brújula”), mes (month), res (livestock), and some nationalities like francés, inglés, japonés, etc.
