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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 — [...]

Good question. And the answer is…there is no easy answer. Even though words ending in a are generally feminine, this is not always the case. Likewise, words ending in o are not always masculine. Five notable exceptions are:
la mano — the hand
el día — the day
el problema — the problem
el poema — the poem
el mapa [...]