Questions about adjectives
Q: Why is it una flor hermosa and NOT una flor hermoso?
Adjectives in Spanish must agree in gender with the noun they modify. Because the noun flor is feminine, the adjective must also be in its feminine form.
Q: Why is it una gran idea and NOT una grande idea?
When this adjective appears before the noun it modifies, it changes from grande to gran. This adjective has the same masculine and feminine forms.
Q: Why is it un libro interesante and NOT un interesante libro?
Most adjectives (e.g.: importante, gentil) follow the noun in Spanish, unless the speaker/writer wishes to highlight the emotional significance of the meaning of the adjective. This is of course different from what occurs in English, where all adjectives precede nouns (e.g.: an interesting movie).
Q: Why is it los carros blancos NOT los carros blanco?
Determiners (los), adjectives (blancos) and nouns (carros) must agree in terms of number (i.e.: singular or plural). If the noun is in the plural, e.g. carros, the adjective and determiner must reflect this: los carros blancos.
Q: Why is it un hombre oportunista and NOT un hombre oportunisto?
Adjectives that end in -ista (except listo/lista) do not have separate masculine/feminine forms so you use the -ista ending to describe both masculine and feminine nouns.