The plural form of nouns in Spanish is relatively easy to learn. There are really only three rules to follow.
1. Nouns ending in a vowel; add s:
mano — hand
manos — hands
señora — lady
señoras — ladies
2. Nouns ending in a consonant; add es:
lección — lesson
lecciones — lessons
profesor — professor
profesores — professors
3. Nouns ending in z; change z to c and add es:
luz — light
luces — lights
nariz — nose
narices — noses
Notice that under rule 2 above, the example also showed a change in written (and spoken) accentuation:
lección becomes lecciones
That is because adding es actually adds another syllable to the word and so a different accent rule applies. You may want to review the lesson on accents in Spanish.
In a nutshell, when words ending in n, s, or a vowel have a spoken stress, the accent mark is used. The word lecciones keeps its spoken stress on the -on syllable, but in the plural form of the word, with -es added, -on is no longer the final syllable and the word conforms to the unmarked configuration.
Okay, that was la parte más fácil. Now you have to know how to add “the” definite article. You see, in Spanish the article, noun and adjective must agree in number and gender. Think of it as a homogeneous noun phrase.
For example, if “the lady” is la señora, then “the ladies” is:
That is, la becomes las, for the feminine gender. The masculine el becomes los in the plural form:
el hombre fuerte — the strong man
los hombres fuertes — the strong men
Do you see that the article, the noun, and the adjective all become plural?
To anticipate your next question, and offer some help with it, visit: When are Spanish Nouns Feminine or Masculine?
Espero que encuentre las lecciones en este sitio informativos y divertidos.