To form a plural of nouns and adjectives that end in a vowel (-a,-o,-i,-u,-e) we add -s:
hermano → hermanos
To form a plural of nouns and adjectives that end in a consonant, we add -es:
alemán → alemanes
Forming a plural, the stress (la tilde) can disappear or (less frequently) appear:
holandés → holandeses
joven → jóvenes
For more information, please, consult this page